TechPowerUp's "W1zzard", who makes the wonderful and ubiquitous GPU-Z software, has taken it upon himself to overclock AMD's R9 Fury X card and report back on its voltage scaling. He accomplished this feat after struggling to add Fury X voltage monitoring and controls to GPU-Z, which he describes as no easy task.
The card was first tested at its 500-MHz stock memory clock, with stock fan speeds. The power limit was set at +50% to avoid throttling. Battlefield 3 was the testing title of choice, running at 4K resolution. W1zzard worked upward in 24 mV steps from stock, recording the maximum stable GPU clock speed at each step. He eventually found a voltage ceiling of +144mV, which was good for GPU clocks of roughly 1215 MHz at a rather high core voltage of 1.35V. With these settings, the full system's power draw was roughly 575W, a staggering 125W increase over stock settings. The VRMs were quite toasty at 95°C, an alarming temperature considering they're under a watercooling loop.
The fly in the ointment here is that all those extra megahertz and watts netted about a 3-FPS increase, which isn't impressive at all (W1zzard does not indicate if his numbers are FPS maximum values or averages.) And that's at the maximum setting, too—more reasonable clocks might as well net next to nothing. He also overclocked the card's memory to 560 MHz, but that yielded just another 1.6 FPS, with artifacts appearing at clocks above that. All that aside, W1zzard says 40 mV is a reasonably safe core voltage increase.
One bright spot was that the Fury X's watercooling setup kept GPU temperatures in check all throughout W1zzard's testing, although he notes that all the extra heat just gets dumped into the room.