I saw an article on a web site that showed benchmarks on several systems and several games, and the only thing that they changed between each test was the Cat versions. Newer Cats seem to do a hair better on 3DMark, etc., but actual in-game performance actually gradually decreased, for UT (or maybe UT2K3, I don't recall).
I've been wondering that myself, I've had a few minor issues with Cat 4.6s, and heard good things about Cat 3.10s mentioned.