When it comes to running Nexuiz on the low-end NVIDIA ION hardware, the Nouveau driver was actually running at the same speed as the binary driver. The NVIDIA ION graphics core on both drivers was running at 450MHz. While the performance between the two drivers was effectively the same, at 1920 x 1080 it was a mere 10 FPS so it's unplayable frame-rates on any 1080p displays.
With OpenArena 0.8.5, the Nouveau driver was putting out 40 FPS but the NVIDIA 310.14 closed-source driver was pushing 51 FPS with the GeForce 9400 graphics. At 1920 x 1080, it was 30 vs. 37 FPS in favor of NVIDIA's binary blob.
With the newer OpenArena 0.8.8 release that makes greater use of GLSL and other GL2 functionality, the performance between the Nouveau driver and the NVIDIA binary driver was now the same.