S!
ATI runs a bit worse than nVidia on IL-2 due the code has been optimized for nVidia. You can clearly see this optimization when you open the console at beginning of track and look at the list of NV stuff there. Then compare to the same list with ATI and it is a lot shorter. So this has an effect for sure.
For 5xxx series card the 9.10 atoglxx.dll in game folder seems to work just fine. I have not tested 4xxx-series, but if getting those numbers seen above I would be happy. The Black Death track is good to give pointers, but does not reflect the actual gameplay. I got higher FPS in more demanding scenes in-game than on the track.
And as stated many times before ATI is working on this, so have to wait a bit as fixing something like this ain't done in a day or two. Some tips below for testing
Some might even try 9.10 drivers for IL-2. But if playing modern games this might not be an option though.
Vertex Arrays can be enabled if setting DynamicalLights=0. No blue hue!
You do not need Polygon Stipple or DrawLandByTriangles => 0.
Disable Catalyst AI, no use in IL-2 from my experience.
Water=0 has big impact on performance with some loss of eyecandy.
Forest=3 is less demanding on your system than 1 or 2 for some odd reason.
Effects=1 is good, using setting 2 is like begging for trouble. Bugged and not recommended, not even by Oleg.