Here's my results with two Sapphire 6950.
Summary: My second card is nearly dead, either due to the shader unlocking or memory overclocking. Read the rest and draw your own conclusion.
The first one flashes without problems. I have tried with the 6970 BIOS and a modified 6950 Asus BIOS, both BIOS works brilliantly. I can run the same speed as before the flash + a little extra on the core. All according to expectations.
The second one flashes without problems too. I have tried with just the 6970 BIOS on this card. Now here's when it gets interesting. Before the flash I had stable 900/1480. To make sure I still have these speeds after the mod I isolate the core and mem and test separately. First the memory, so I lower the core to 800 and set the mem to 1480. I load a test replay in Starcraft II (which I have found to stress both core and mem more than Furmark and alike) and after 15 min my screen is divided in two, no artifacts, but two screens. Weird, and nothing I have seen while testing on my two cards so far. So I lower the clocks to 800/1250 just to try and isolate the cause. Could be the unlocked shaders, but it could also be the memory clock. With the stock clocks I run some replays without any errors. So I try to up the mem freq to 1450, no errors. 1470 then, 1 short replay playes fine, but a longer one blue screens (0x00000116) at the 15 minute mark.
Last time I had this 116 bluescreen was when I unlocked my other card, upped the mem freq to 1550 and stability tested in Starcraft II.
So, no biggie I think, I'll just reboot and lower the memory frequency a little. Terror and horror! At the boot screen (where CPU and memory speed, etc. is shown) I notice some white dots a boxes spread all over the screen. The windows load screen looks normal, but upon entering Windows I get the same bluescreen. Switching BIOS using the jumper made no difference at all. Now, booting in safe mode gave me no trouble. But flashing back to 6950 was a no go in safe mode, so I uninstalled ATI-drivers, rebooted and this time I could enter Windows without using safe mode. But the screen was tearing all over the place and 80% of the screen was green. Flashing back to my original BIOS caused me no trouble. It didn't help anything though, from the first POST screen everything was tearing and artifacts all over the place. I couldn't confirm the shader count had reversed to the original state, so I tried to install the ATI-driver again, but ofc it caused a bluescreen upon entering Windows.
So I installed both graphics cards, the working one in the primary PCI lane ofc. With the crossfire bridge installed I would get different bluescreens entering Windows. First a 0x000007E, then on the next boot a 0x00000116 again. Uninstalling the crossfire bridge I could enter Windows and using GPU-Z confirm that the secondary card only had 1408 shaders and thus should be completely restored to it's original BIOS state.
Now, as I see it, this could be caused by one of the three: overclocking the memory, unlocking the shaders or unlocking the shaders and overclocking the memory.
Anyway, just posted this to keep you updated. Keep up the good work.