Starting to see a trend with the AMD Catalyst driver performance? Switching between Compiz / Metacity / KWin window managers is not causing a performance hit, but when using GNOME Shell with Mutter it is a very troublesome pair. It is also under the Catalyst driver on GNOME Shell where there were the icon issues, etc.
When running World of Padman on the proprietary NVIDIA driver, there is finally a test result where the KDE desktop does not come out in the lead, but in fact, for this open-source game it is the slowest. However, with the NVIDIA blob, all five tested configurations had 400+ FPS frame-rates, so it is not much of a big deal.
For the open-source drivers, interestingly for this workload the frame-rates under GNOME Shell + Mutter were higher than KDE + KWin. The fastest for both though was using the GNOME 2.32 desktop with the non-compositing Metacity.
One item to note about this test run is that the resolution was set to 1280 x 1024 and not the panel's native 1920 x 1080.