Lightsmark is a cross-platform OpenGL demo that has been rather good for benchmarking since 2008 and supports more features than many of the open-source games thanks to its real-time global illumination, penumbra shadows, and other lighting features. However, as sad as it may be, Lightsmark 2008 on Windows was unimaginably faster -- with 100+ frames per second differences. Ubuntu 10.04 only came out ahead once and that was with the Athlon II X3 system and its Radeon HD 4670 graphics when running at 1920 x 1080 where it had an odd performance spike similar to the FirePro setup.
Worth noting is that similar to the OpenArena numbers, the dual AMD Opteron "Shanghai" setup with its FirePro V8800 graphics was rather sluggish with its proprietary ATI driver until it was running at 1920 x 1080, the same resolution as the X.Org Server. It is looking like this may indeed be a Linux-specific bug within the proprietary AMD graphics driver stack when running at the X.Org Server's standard mode.