Poor FPS? how poor like 20 FPS?
Try these options:
1) Try the 12.6 BETA driver (scroll down the page after you click on this link)http://support.amd.com/us/kbarticles/Pages/AMDCatalyst126beta.aspx
2) You can go into the console/command of CSS and type: max_fps #
So if you want put it at max_fps 120 so that it will not cap the frames.
How to Enable Console
Go to Options->Click Advanced near bottom->mark the box Enable Developer Console
Then go back to the game and press the (~) key, top left near the ESC key.