Starting off with the CoreBreach racing game, the fastest desktops for running this OpenGL racing game were Xfce and Openbox, which don't rely upon compositing by default, while slightly behind those two was LXDE. The slowest desktop environments were KDE Plasma and the GNOME Shell.
The Nexuiz game benchmark ended up being bottlenecked elsewhere in the OS stack so there was no real change between the different desktops on Ubuntu 13.04.
To no surprise, Xfce, Openbox, and LXDE were the fastest. Unity and GNOME Shell led to having the lowest frame-rates.