ALL the 5k series support it. Just some have it turned off in the bios. But a quick edit or a flash and everyone can have the voltage tweaked in afterburner for example.
Have you changed the BIOS? What cooling do you have on the card as when you increase the volts the temps on that card will rocket. STD cooling will not be enough.
Search google for something like Asus 5xxx bios (insert card of your choice), download the .bin file. Boot into a convenient copy of dos, run awdflash.exe or similar and follow the instructions.
No liability is taken for the bricking of your card.