ok, this is definitively weird.
First of all, you can force atiwinfash to flash like you do for the dos version ("atiwinflash -f -p 0 mybios.rom")
So I tried to flash the bios resulting from just opening and saving my extracted bios. I was expecting a no change in behaviour. Well , not so
Initial bios regulates arround 82°
Saved bios regulates arround 71°
So there is something in your bios 'changes' that the (or my ?) 4870 does make use of
Initial bios is
Here
Saved bios is
here
Also, just to confirn, I have after flashing the 'saved' version, I have re-extracted with GPU-Z, to make sure the winflash wasn't part of the issue, the two file are 100% identical, so I don't need to go to dos
3d_jc