I had a bit of trouble flashing my Asus card. (2Gb ref card, slight O/C).
I tried to flash a 6970 BIOS to it, even used an Asus BIOS, but I got the subsystem mismatch error.
Only thing I could get to work was to use RBE to mod my cards own BIOS.
Got the shaders unlocked, but still at 6950 speeds.
Card seems to be working fine at the max O/C AMD Overdive will allow, 840/1325.

Although I have to say it hasn't made a massive difference.
Heaven benchmark went from 30.0fps with a max overclocked 6950, to 30.8 with max overclock and unlocked shaders.
So the extra shaders gave me 0.8 fps! I was kinda hoping for more!