Lightsmark has regressed badly in Mesa just recently. It is failing with Nouveau Gallium3D and with the R600g driver it ran under GNOME2 + Metacity and KDE. You may note that the R600g Lightsmark frame-rate is much higher than it usually is in Phoronix articles, but it is not due to OpenGL driver optimizations. It is actually due to recent regressions with many of the textures no longer rendering in Lightsmark and it mostly just being a black surface. The proprietary Catalyst performance was the same across the four tested desktops, but it failed to run under GNOME Shell.
With the proprietary NVIDIA driver, the KDE desktop was again the fastest while GNOME on Metacity ended up being the slowest and the rest of the configurations falling right in between.