actually I don't see anyone over at xs flashing their 4870s for clocks. Perhaps they can't get it stable over 790mhz (ccc limit), and since no one knows how to increase voltage yet that's the limit.
If you want to try download some stock bioses from the techpowerup database, use RBE v1.11 to edit them and the latest version of atiflash to flash them. I would only use those tools as the 4870s have a much different bios than anything else previously...it's 128kb instead of 64kb because of the additional gDDR5 instructions