In terms of "does it work" - short but silly answer is YES. There is a price for it however. That price is the fact that the driver ignores core voltage settings on all power stages in the custom bios which in this particular rig results in 200 watts extra power draw from the wall. I was unsuccessful with trying to set my own voltage in claymore, wattman, watttool and afterburner - driver completely ignored given settings.
I must take back what I said earlier. Spent a bit more time with the AMD beta driver which initially behaved very similar to the one from the 1st post. I was able to get normal power draw with better hashrate.
My problem initially came from the modded bios. As I underclocked and undervolted the cards in the bios then this beta driver makes its own assumptions of what the core voltage should be above the highest clock defined in the bios. This was not the case with 17.10.3211.1031 driver - on which the card preserved the highest bios set voltage on all core clocks above.
https://i.imgsafe.org/98/98d133c919.pngThis was setting 0.9375v core voltage on clocks 1125+. On AMD beta driver setting anything over 1125 was causing the clocks to fluctuate and the power draw was ridiculously high.
However when I stick to what I've set in the bios it works as it should.
https://i.imgsafe.org/98/98d5c4da1b.pngSo it looks like I will have to modify the bios for higher clocks if I want to keep the voltage at this level.
