I'd think that if on the original bios of the 5870 updating it would be for the better.
Just a FYI from here down!
The stock bios kinda sucked when released. I flashed my 5850 with a Asus bios way back then! Good for 1000/1300 afterwards....Full water block to tame the temps as those vregs get very, very hot when pushing the card to it's potential
I wouldn't suggest trying for some crazy high overclock if you flash it. Well you can overclock but keep an eye on the temps. The vregs get hotter than the core by alot when pushing the card.
If I remember correctly my 5850 was flashed with the modded 5870 bios version tho. Gives a little xtra vram voltage if I remember correctly.