Download files

Create DOS bootable USB drive

Copy files to USB drive

Copy atiflash.exe and vbe7 exe to the bootable usb drive.

Copy vBIOS from GPU

Boot from the USB drive.
To save the bios from card 0 to file 0.rom run
atiflash -s 0 0.rom
If you have more than 1 card rerun the command with increasing numbers.

Modify vBIOS with VBE7

Write vBIOS to GPU

Boot from the USB drive.
Write the bios by running
atiflash -p 0 0_new.rom

Stable voltages for me

CardPart noVoltage
Sapphire AMD R9 280X DUAL-X OC 3GB GDDR511221-00-25G1131mV
Powercolor TurboDuo R9 280X 3GB GDDR5 OCAXR9 280X 3GBD5-T2DHEG/OC1100mV
Gigabyte Radeon R9 280X OC 3GB GDDR5 OCGV-R928XOC-3GD1094mV