Hey there
"Latest version" is not very informative, such things have numbers for good reason.
Now, that said, there isn't enough data here for a differential analysis, nor any explanation as to why you are thinking, OC or BIOS as candidates.

Assuming you didn't change either of those (OC/Bios) when you migrated your GPUs, from old rig to new rig, (did you?, as you mention 6 GPUs in old rig, and 9 in the new rig), so are 6 of those GPUs moved from old to new rig, and now seem to be hashing slower?

What make/model GPU?

Also, what is far more relevant here is, new motherboard, new ram, new cpu, new OS, new install of everything, and assuming you didn't change BIOS or Claymore config, I'd strongly suggest investigating all the former, before turning the spotlight on the latter.

Some things to consider.

What OS version on both rigs?
IME Win10 Home version out performs Pro in almost every respect, gaming or mining. For one, it's loading a LOT less MS-crap that most people don't need. (If you're not joining a domain, you probably don't need Pro). The number of services is nearly double on Pro, and again, probably you don't need any of the pro stuff for mining.
Mining and stability go hand in hand, and keeping the OS as lean as possible, updates disabled, (set all NICs to metered connection), and manually control/manage how/when you plan on updating, (if at all).

FYI, I also have one rig on the B250ME, and have no issues with different hash rates given same GPU/driver/claymore ver. (I also moved GPUs to that rig, and from it, as I was able to consolidate same make/model/ cards on it) (mixed mem type still, and no issues).
This has been so since day 1 with the B250.
Where the B250 did become a pig, was getting it stable with the 12th GPU, and worse still with the 13th.
That said, spot check, clocking through 75 hours, (claymore 11.7, Win10-64Bit-Radeon-Software-Adrenalin-Edition-18.3.4-March23, Win10Pro 10.0.14393) which is not a record, but I had to reboot last Sat), but the B250 is running well these last 4~5 weeks.
(13x RX580) Sapphire, mix of Hynix/Samsung/Micron2, all on custom BIOS, all hashing >32.

But as you asked, I HAVE seen some issues with ATIWinflash.exe, I've certainly taken some shortcuts, and regretted it. If nothing else, shortcuts add an element of doubt when analysing issues later on, and in some situations, you're unable to quantify those, (doubt remains).

When working with networked systems, by "networked" I mean, multiple computers, (gpus), in a single entity, it's paramount to be systematic, anal always helps :-)

Some tips.
Note the serial numbers of your GPUs, backup the BIOS before you do anything, (use the serial number in the bios file names).
Start off with stock BIOS, (for some days), so you establish a reasonable baseline of data.
GATHER data, write it down. What is "normal" for each etc
Make sure you know which GPU number (in Claymore, in GPU-z, in Radeon-settings), is which, and that way, you know what you are changing, (and can revert), and most important of all, DIFFERENTIATE when you see issues.

I've seen stock BIOS on RX580s, as low as 18MH/s and typically no higher than 29MH/s.

If you mod the BIOS, work on only ONE GPU at a time, version your BIOS edits, (CardMakeModel_SerialNumber_Ver) or something to that affect.
Key point is, make SURE it's clearly identifiable.

Get yourself a METHOD, you are going to end up managing and maintaining a complex system, there are a LOT of variables, so with a standard method, you can at least hope to rule out all the dumb human elements.

Easier said than done, and one should consider "method" a work in progress, refine that as you go along, and maintaining rigs DOES get easier with experience, and systematic analysis.

One thing about fine tuning BIOs: Initially I was struggling to break 30MH/s and made some mistakes,  identified dead ends, had some cards that were real pigs, but actually turned out far better than I expected, even on more than one occasion considered returning.

Try to use all the same make/model/mem-type of card if you plan on a small operation, (<10 GPUs).
I don't say this to cherry pick, or exclude any vendor, but simply because with a small number of cards, it's very hard to know what ""normal" is for them. If you have only 1x Asus RX580, how do you know if it's good/bad/ugly? If you have 2 or more, you can at least hope to make some comparisons.

Also, don't believe everything you read online, (and feel free to ignore me :-) but MAN I read a lot of utter RUBBISH online. In fact, finding the accurate data is the toughest challenge.

One example, that still puzzles me, (note: puzzles me, not saying it's rubbish), is how many people rave about Samsung memory. In my experience, Samsung has been the best out-of-the-box GDDR5, but also the toughest to fine tune. In fact, I'd suggest it's not even worth trying memory strap edits with it.
Drop the power consumption & core clock, up the memory to 2250, and you should be seeing 32.5, and stable. I wasted a lot of time trying to better that, only because I didn't see a big jump, and due to hype, assumed Samsung was the fastest.
By the time I got my hands on my 2 and only Samsung GPUs, I had Micron running super stable at 32.6, up from 29~30 stock, and (mistakenly) figured I could see similar gains with Samsung.

My present conclusion is, Samsung mem is already pretty tightly dialled in, so as an out of the box card, it's certainly the best I have seen, but it certainly is not the top performer in my rigs.

Some notes on BIOS editing.
Make sure you have a versioned stock bios file saved.
Use THAT file for editing. (Do not use BIOS from the net).
Depending on the card and editor, it's highly likely there are other bios elements that are not displayed or editable in the bios editor. Who knows what you change if you flash some random BIOS from the net.
Make sure you have only a single card connected when you flash.
(For SURE, I have done a enmasse deploy to 3 GPUs on multiple occasions, and in recent weeks identified that as a 100% confirmed issue). 2 different rigs, 1~2 cards started to show got incorrect share, after some hours.
What was weird was, those were all cards that were mass-flashed.
What solved that?

Several things.

Shut down rig, bleed down power, remove all but 1 GPU (usb cable), boot up, flash that one card, (I used the same BIOS ver for that same card as last flash), do NOT click ok to the "you must reboot message", but actually shut down, and bleed down the power.
I have a suspicion the dual BIOs cards are capable of holding the BIOS in DRAM and not actually booting to the newly flashed BIOS.
Sure, that will depend on the architecture of the card, but BIOS is typically EERAM, or some similar flash ram type chip.
The card is NOT going to use this (new program), unless several things happen.
A flag/trip is set, (by the flash app), to reload BIOS, (this seems to fail sometimes with Winflash), but for sure, what will force the card to load the new BIOS on next boot, is removal of power, (bleed-down), as this clears the DRAM, and leave the card no choice but to load from EERAM.

Did you notice how slow it is to program BIOS? Those are usually 256 or 512 KB chips. That is KB, KB, nothing these days, yet flashing takes nearly 1 minute for 512 chips. Sure the write speed is slower, but even the verify is slow, and this is because the eeram is not intended to be fast, or used much, (written to), but is depended on to safely store data, without any powered backup.
What typically happens is, as soon as power comes up, the dram content is crc checked with eeram, if ok, card boots, if not, it loads from eeram. As this happens at power on, before os init, that causes no delay in booting. What the winflash does is cause change in crc, and that SHOULD trigger the card to load the bios from eeram on init, but this has certainly failed for me.

I saw no change in card performance after flashing. Stopped claymore, opened winflash, saved BIOS, and sure enough it WAS new, so flash was ok. I shut down, bled-down power, rebooted, and THEN I saw the improvement I was expecting.

My point is, that winflash is only able to read the bios from the eeram, NOT from dram.

Basically you can think of it much like your computer, eeram=hdd and dram=system memory.
This architecture is not so by accident.

The purest will also say you should not use winflash, but rather boot to a very light os, (Dos), and flash from there. This is actually very good advice, but I decided I'd use that as the fall back, and at least start out with WinFlash, as it IS vendor provided after all.


But for sure, it has it's use case, and multiple flashing is a time-saver I've decided is not worth the gain.

But at the same time, I've yet to see those issues insurmountable.

OK, last work on OC.
OC is pushing your luck, if you see issues, remember, you are pushing your luck.
As for the silicone lottery, at least with Sapphire cards, having built rigs for a few people now, I've only had 1 card out of 30 something, that was clearly a dud.
BUT the only way I can say that with certainly is because I documented all the 30+ other cards, (same make/model/memory), and no matter what I did to that one card, it was not stable with any oc at all.
Is that the silicone lottery? I don't think so, I think it was simply a QC issue.
I could be wrong, maybe someone with 100+ cards, data, and experience can chime in here.



OK< last thing.

Did you try setting the dcri?

New rig has 9 GPUs, cmiiw.

DCRI will be different for sure.
Disable dual mining for a start, (pointless, no profit unless you have free power, and even then dubious imho).
Try the "z" key during solo mining, and let claymore find the optimum.
If those numbers are very different from your dcri value, (>2) then I'd suggest at least picking the mean for your config file.

What happened to your hash rate after "z"?

Side note, and no disrespect to Mr Claymore, but usually I find a better dcri value, by using the -+ keys, and set same for all gpu.
But for sure, the z key is a nice feature and great tool to get you 95% optimised and quickly.

Also, set -y 1 check your console during init and make sure you see both lines below.
 
All AMD cards use Compute Mode already
CrossFire is disabled already

 
Otherwise, open admin console, start your batch file as usual, wait for the "all cards now use computer, please reboot"
Reboot, and run claymore as normal, (no need for admin).

THIS IS THE BEST FEATURE from Claymore of late, if you have more than 5GPU, otherwise it's hours of lame AMD-settings, and reboots, to configure each GPU.

Finally, if still no luck, post your batch file, and some specifics on hardware/versions etc.

Good luck.