So if I understand it correctly, best efficiency(W/TH) is mainly based on voltage, right? Therefore I should try to run them all on 7.9 and tune the frequency?
From what i understand, after you lower the voltage, you lower the speed until the asic board starts hashing properly again. This is a bit of a trial and error and needs lots of reboots, and each board (chain) seems to like different settings. Its the opposite of overclocking, where you set a high speed then increase the voltage until it hashes again, and hopefully don't get too many HW errors (or melt your chips).