I wanted to report partial success flashing my Sapphire HD 7950. The model I have is the "Dual-X fans w/ Boost", or SKU 11196-16-20G. This is one of the models with a non-reference port layout, with 2xDVI, 1xDP (*not* mini-DP) and 1xHDMI.
I tried several times to 'roll my own' ROM, and each time met with failure. Initially it was because I used device ID 697a, but even after correcting for that I couldn't get a boot screen. Then I found that my ROM had a checksum of 0x0, which was causing the python script to fail. So I had to change this line in fixrom.py:
self.data[-1] = chr(0x100 - sum)
to this:
self.data[-1] = chr((0x100 - sum)%0x100)
Even after making these changes, the card would not display boot screens and would always show as '7xxx' in 'About this Mac.'
Finally, I combed through the thread looking for someone who had had success with my model. I found Post #441 by JoepVK where he reported that it worked, but had issues with DisplayPort. I flashed his modded ROM and had a similar outcome.
I have similar issues with HDMI, but DVI works fine for me. That's good enough for me to move on with my life. Thanks @JoepVK...