Ya mainly 2-3%, any firmware that asks for more should be avoided, and I don't think it "periodically changes", at least not for Vnish, they claim "Parallel mining" which seems to be the case when you observe the reported hashrate on the pool, it doesn't seem like it discounts at all, but you do see that hashrate being equal to what shows on the pool minus the fees so a few gigahashes are going to be lost


That's great, I wish Bitmain would do something similar to Avalon, you know none of us really want to pay those dev fees, but the stock firmware is too bad, the custom firmware devs knew that and they know there would be a huge market for custom S9 firmware, one reason they didn't touch the Avalon is probably the fact that Avalon users don't really need a custom firmware since the custom firmware does what they need.


Cost per terahash won't favor Whatsminer M21s, it's a newer generation gear, not in the S17 range but somewhere in between, it's about 56w/terahash, but you should keep in mind that most gears mentioned here including my own pick of S9 are gears that have been around for way too long, the last time Bitmain made an S9j/S9i was probably around 2018 or very early 2019, so most gears in circulation are  AT LEAST 2 years old and above, the average would be 3-4 years, a very critical age as far as I am concerned,  pretty soon most S9s on planet earth will either die or go through a major rebuild which might not make sense in terms of cost.

In other words, IMO, you should hedge yourself against the lifespan of these old gears buy acquiring a few newer and more efficient gears, while your 2.5 cents Khw rate is very good, it's isn't perfect, so eventually, you will be kicked out by the efficient gears, so maybe 20-30% of your hashrate should be in the 60 or less w/terahash.