Memory damage is what would woryr me more; the 6970 bios uses higher frequencies and higher voltage for the memory chips.
Since 6950 cards use lower grade memory chips they will fail over time.
Instability might be a blessing, clearly showing the user how unhappy that the reflashed card is feeling.
If you have a 1GB version, you can forget about flashing the bios as all 6970 cards are equipped with 2GB of memory.
This is why alternative methods were developed, including the RBE method and the approach where you save your bios to a file and use a ready-made script which changes just the few bytes responsible for shader count.
Both these methods work for reference card designs only.