Clock-For-Clock, Nouveau Can Compete With NVIDIA's Driver
Starting things off with Nexuiz, for the GeForce 9500GT and GeForce 9800GT the frame-rate of this open-source OpenGL game was matched between Nouveau and the NVIDIA blob when clocked at the same frequencies. The 9800GT was even faster than the NVIDIA binary driver when both drivers were controlling the graphics card at 399/399MHz. Obviously though the NVIDIA binary driver still whens when controlling the hardware at its rated frequencies.
The OpenArena results were similar to Nexuiz where when the Nouveau and NVIDIA drivers were running at the same (lower) clock speeds, the frame-rate was competitive between the two drivers.