My rig: 5 xfx 470, core 1100, memory 1975, 141mhs and 685 watts in the wall.
How you did the lower the voltage on xfx ?
I can not control the voltage on my xfx`s i did try with wattol, afterburner, trixx... no one work for me
I do not have this kind of card, so mine is just an advice.
Have you tried to modify it in the ROM directly?