What fps do you have ?
On my AthlonXP2400 / Radeon 9600 I have some 22 fps at normal details, 12-13 at High details, but I use Omega Drivers. Before that with Cat3 I had half of this.
Funnily on my old Athlon500/GF2MX I managed to have same fps ! ...
It just seems like the Radeons don't do well with DomII, especially the Pro's. I have a 9600 pro and get 8 fps high and 13 fps medium, so I have to play with low for 25 fps. My brother barely did better with his 9800 pro.
I did try the omega drivers, but they caused a small but noticable performance drop on everything I tested, including DomII. Needless to say I was unimpressed and removed them and reinstalled my old drivers.
It would be nice if Illwinter wrote a direct3d Version or optimazations for radeon cards, but although I know nothing about 3d coding I still think it would be too much trouble for them to bother.