I have same performance issue with opengl on both windows and linux. With opengl on lowest possible settings i have about 30 fps in game and its drops below 20 in teamfights but with directx on mid/high settings i have about 50-80 fps and it is never drops below 30 even in teamfights. I am using hp 630 laptop with core i3 cpu and amd 6370m card with latest catalyst driver.
On the other hand, AMD free driver strategy seems to be working. When Squeeze was released, I tried their free radeon driver with a mesa version I got from the experimental repo (┐maybe 7.9?) and I was able to run the game!! It had some small problems, mainly the character and enemies were missing, but everything else was rendered correctly and fast enough.
I have much better experience with OS drivers. Native UT2k4 isn't playable with catalyst, because it lags terribly, but it works nearly perfect with Open Source radeon drivers. It's just a bit too slow on some maps, but there's no lag.