That could be due to game implementation details; I'd only buy your line there Kano, if you could prove that it was as fast or faster on the proprietary drivers and the FOSS Radeon support on G200 and G300/400/500.
(Not that I don't think you're right- I just don't have any basis as I've not measured and I've not seen anyone else do it either...

)
If it's slower on most or all GPUs, then there's something with the game itself. If it's only slower on a few GPUs, then it's definitely driver issues.