Well, I can confirm another success with this mod - I got a Powercolor X800 pro vivo, and after running the various 3DMarks, I flashed the bios using the modified flashrom program. I used the Powercolor X800XT-PE bios, just because my card was Powercolor to begin with. After I restarted and reinstalled drivers, I opened ATITool to confirm, and now my card is running at X800XT-PE speeds (520/560) with 16 active pipelives. The highest temperature it's ever been to is 67C, and that's at 66% fan speed, using the automatic settings. All this with stock cooling. Before I bought it, I had some concerns about the number of people having to buy 3rd party coolers, but I looked at pictures of the X800 pro vivo and the X800XT-PE, and they use exactly the same cooling anyway, at least the Powercolor ones do. I figured then, that with a bit of luck, my card should run at XT-PE speeds with stock cooling - and it did, so I guess I was just lucky.
Good luck to anyone else who tries this mod.