i was playing austria last nite, and in a replay of the start looking down the straight i was getting as low as 30fps when the dirt offline was kicked up. With that in mind i reduced DSR to x2 (instead of x4) which helps considerably, only drops to high 40s now. Also one thing that helped is that i used to run 22 cars (myslef included) now i just run 19, and that helps the fps, as it seems to allow more headroom. Also some tracks seem to be worse for the dust than others. But drs x2 with x2 AA still looks pretty good and still sharp..
I wont lie, GSC isnt the best optimised game out there, but with reizas small team i will allow this. Considering i can run AC full at 60 fps... its a small sacrifice to drop a few cars or gfx to get this playable.
the end of the day its worth it, and races miles better than AC, does and ever will imo.