Of course he's going to get occaional slowdowns though - it's an extremely demanding game on full settings, it's also quite buggy and IMO not particularly well optimized. And one persons slowdown is another persons liquid smooth. These are entirely subjective terms. Some people think 20 fps is fine, others can't stand anything less than 60 fps. You also can't read anything into how just one single system performs, there's a million different factors that could be adversely affecting performance. Unless you get hard numbers you can't reach any meaningful conclusions, and the hard numbers say 320mb isn't a limitation in Bioshock, and that the 8800 GTS 320mb is around 50% quicker than X1950 Pro (which isn't close).
I do know about that memory bug, but haven't run into it myself yet in the 6 weeks I've had my 8800, and that includes a 7hr SupCom game and several other 5+hr games, and playing R6Vegas all the way through in 3hr+ sessions. Not saying the bug doesn't exist, but I don't think it necessarily occurs that often.
Btw I'm not denying an 8800 GTS 320mb will struggle a bit with Bioshock in 1680*1050, I said as much in the post that started this debate, but the 640mb version would struggle equally, and an X1950 with any amount of vram would struggle significantly more, it's just a very demanding game shader-wise, it uses masses of pixel shader effects. An 8800 GTX/Ultra would struggle in 1920*1200.