Ha... at your own risk! I'm only giving you the info I observed in the past, and I didn't look into it further to get any definitive answers. I'd suggest doing your own experiment to verify what I saw. You can run stock firmware immersed if you have some fan spoofers. If you run stock in the same setup and the temps come out to be 10degC lower, then make your own conclusions...

The chips were designed to be run hot, the temperature delta to ambient is needed to move all that heat with air cooling. So you can actually run into some problems if you run too cold, timing on the ASIC design is very complicated and is temperature-dependent. So if they designed the chip for say 70degC to 90degC range, it may not meet timing requirements at 65degC for a given frequency and voltage.

I'm sure there's a sweet spot that would be below 80degC, but you'd need to experiment on that to find out. My point about the 80deg target on factory firmware is only that Bitmain seems to think that their chips are happy at that temperature.