Windows 8.1 Bios Flash || Matrix 5870P

So I have the Matrix 5870, wanted to do some overclocking. The main issue with this card is that the bios had a maximum voltage of 1200mv, so I flashed the bios from the Matrix 5870 bios, to the Matrix 5870 Platinum bios.
All was well and good, until I began to get artifacts, then I found out that the voltage on the Matrix Platinum is still 1200mv. while I was overclocking I upgraded my OS as well, so the Radeon bios flashing tool doesn't work with WIndows 8.1.
So I need help flashing my bios, on Windows 8.1. I have written a custom bios, with correct voltages and clock speeds to suit. I just need to be able to flash it.

New GPU isn't an option.
Windows 7 doesn't work either, since the bios flash.

Relevant info:
CPU: AMD FX-8320 @4.20GHz, 1.4750mv.
Mobo: ASRock 990FX Extreme3
RAM: Corsair Dominator 6GB tri-channel @1600MHz
GPU: ASUS ROG Matrix 5870 *Matrix Platinum bios*
OS: Windows 8.1