[Graphics Issue] Ground Cover and Shader Detail FPS Hit on Radeon HD 7950

[HazzaP555 0]

I have recently installed an AMD Radeon HD 7950 in my Mac Pro (running in Windows 7),  replacing my HD 5770. I had modest performance with the 5770 in Dirt Rally, and with the 7950 everything is set to ultra. However, ground cover and shader detail seem to hit the frame rate irrelevant of the setting, i.e. the drop in fps is the same for low shader detail as it is for high, likewise for groundcover. The game runs at super stable 60 frames with everything at ultra except the two mentioned features. They can only run at the lowest setting.

Are there any other similar cases out there?

Specs;
Mac Pro 2,1
Windows 7 64-bit
Intel Xeon 5150 2.66 dual core (x2)
AMD Radeon HD 7950 3GB
8 GB Ram