I have same problem, when using AMD Proprietary, but Open Source provide better fps and less hiccup.
Here some screenshot with "net_graph 1" showing fps rate.
Proprietary (fglrx-update): https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-frc3/1048632_10201400201389911_1820109522_o.jpg
Open Source (X.Org Gallium 0.4 on AMD BARTS): https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-ash4/1026048_10201400201429912_712539590_o.jpg
I have never been able to get any kind of good framerate or latency with my Radeon HD 7950 using the latest catalyst drivers or prior. Open source isn't an option because it triggers enabling LLVM Software Rendering with my desktop which is not great at all because I also use this system for a lot of CPU-heavy tasks. FGLRX itself has a lot of crashes with the Steam client and unbearable framerates in all games. I'm lucky enough to even get 19FPS in TF2. It's also not a power-related problem as I always run the card at max power and beyond.
With latest 13.8 beta drivers this greatly improved. Now it shutter few seconds after start of level. After I look around 360 degrees to force draw whole level it seems quite smooth. The lags are greatly reduced.
Sadly the beta drivers are not usable.
13.4: massive mouse lag and slowdowns (Source games, ETS2). Games like Anomaly dont start
13.6: mouse lag like 13.4 in Red Orchestra or Awesomenauts + grapic glitches in i.e ETS2
13.7: graphic issues in Euro Truck Simulator 2, X-Plane does not start, Souce game very laggy. Yes its better but far from good. Its not only source games, ETS2 laggs too!
13.8: memory leak + graphic issues (ETS2, X-Plane doesnt start)
To sum it up: The only thing resolved is the mouse lag. Latest beta causes memory leak.
Games don't start and if they do its likely to see glitches and massive slowdowns.
I wonder myself if i can purchase AMD products in future.