That's weird...
I haven't BIOS moded my card, so I would suggest a clean install of your OS (win10? That's what I'm using) along with changing to the default BIOS on your GPU. I'm on 21.4.1 aswell.
Pcie 2.0 has half the bandwidth of the 3.0, so I'm not sure this would be the reason for getting less than half the hash rate of what I'm getting with a weaker card without BIOS mod.
Worst case scenario, try to get a used MOBO with pcie 3.0 if you plan on mining for the future, my rampage 3 extreme is going for around usd 200 on ebay in the US and you could in theory use it with more than 4 gpus without much hassle (i only have one).
I'm on W7, trying to avoid 10 if possible. My install discs are 90 mins away, so I'm not doin that right now lol.
And I'm on stock BIOS and physical switched to mining mode, and checked both BIOS to be sure they are correct.
PCI2 2 is prob the case why it's not running fully, I'd just like conformation.
Like when I first got it, I tried playing the reboot RE2, and it wouldn't run worth a crap, so, it's a sign I should have noticed.
When this computer was new, it was a top of the line AVID edit machine, but that was like 08 or 09, so it's been a while. The only reason I'm on it now is my half as old computer bit the dust suddenly, so I'm kinda stuck. I didn't try any games on the old one btw, so no benchmark there either.
I have some coins, if only Doge could hit like $2 ea, I'd take 1/4 and build a new one lol. (well, 2, one to mine and one for me!)