i have some asus R9 270(non x)s with the cuII cooler or whatever... same.. when i enter in values running, or spec them in cgminer config, it still displays in as original volts, but doesnt report any failure, also heat and hash are unaffected.
its there some trick to dropping the voltage with these? linux.
Unfortunately the amd drivers in linux don't allow you to modify the voltages through software.
BUT there is a guide linked in the main post that makes it pretty easy to do the undervolting via modifying the card's bios.