That's the question that the Phoronix crew set out to answer in this article; at least on the R500 end of things.

"However, these changes really have not affected the performance as it has stayed the same for the most part with minimal improvements. In this article today we are going back to test the original 8.24.8 display driver and compare it directly against the latest driver available as of today, the 8.40.4 fglrx release."