Chris Wilson, though an employee at Intel's Open-Source Technology Center, from time-to-time does deal with AMD Radeon and NVIDIA GeForce hardware due to his involvement with the Cairo library and other open-source Linux graphics work. With the Cairo vector graphics library he works on a lot, which in turn is used by projects like GTK+ and Mozilla's Gecko and WebKit and Mono, he's run some new tests of Nouveau and NVIDIA.
He's publicly shared his test results from Nouveau and the NVIDIA 304.64 and NVIDIA 313.18 drivers in this blog post. "The Nvidia ion GPU was released a few years back to cater for the low power netbook market as a substantial upgrade for the anemic Intel GMA950 (or 945gm as it is better known to us) that shipped as the integrated graphics processor in the first Atoms. Those made quite nice machines with long runtimes, a touch on the slow side compared to the full grown laptops, but actually quite comparable to the current generation of premium tablets many years later. People still use these, and I get the occasional request to see how well they perform everytime Nvidia releases a new driver."
His conclusion? While the Nouveau driver is generally slower than the NVIDIA binary driver, the NVIDIA blob for certain operations sees dramatic slowdowns. "Strangely, the nvidia driver fares worse on average than the nouveau driver, despite it having substantially better performance in several cases. This is because the nouveau driver is, at least, consistently slow, whereas the nvidia driver also experience a few severe slowdowns and so has a much more mixed set of results."
Last week I happened to run some OpenGL NVIDIA ION benchmarks of NVIDIA vs. Nouveau, which is also an interesting read for those more concerned about 3D performance than 2D.