With Lightsmark, an OpenGL lighting benchmark, the frame-rate went from 121 to 73 FPS when simply upgrading the driver.
Lastly, with VDrift, the frame-rate went from 20 to 17 FPS.
At least in select software/hardware configurations the NVIDIA 256.25 Linux display driver does carry some hefty performance regressions. Even with games like OpenArena that are not demanding at all of the very recent NVIDIA GPU there was major performance setbacks. Fortunately, however, the NVIDIA 256.25 driver is their first beta release and these major performance problems will hopefully be corrected before the 256.xx Linux driver series is officially christened in the coming months.