I thought I share this moded bios for this Gigabyte model (http://gigabyte.com/Graphics-Card/GV-RX480G1-GAMING-4GD#kf
Stable hashpower 29.3 MHS/ETH with 840 MHS/DCR stable. (-dcri 29)
Voltage offset depends on ASIC quality. Some can take -22 others -12, -10. Just play around with settings. You can go higher with core and mem clocks it can go faster, but it is starting to give memory errors in HWINFO above these settings.
Just flash with atiflash. Use older non RELIVE drivers.
if you have questions feel free to ask.