With the Linux 4.5 kernel bringing PCI Express link speed changes and other alterations to the open-source NVIDIA (Nouveau) driver, here are benchmarks comparing this unofficial open-source NVIDIA Linux driver on Linux 4.4 and Linux 4.5-rc3 compared to the proprietary NVIDIA Linux graphics driver.
The Nouveau changes for Linux 4.5 aren't particularly dominating, but with the PCI-E link speed work and other changes, it's possible there could be some performance improvements to find. The Nouveau re-clocking support remains disabled by default and with GeForce 600/700 Kepler graphics cards your mileage may vary for how well the GPU core and video memory are re-clocking... For this Linux 4.4 and Linux 4.5 benchmarking, I re-clocked all of the Kepler cards tested. If you are upgrading to Linux 4.5, be forewarned the Nouveau re-clocking steps have changed for going about this manual process.
Unfortunately for Linux 4.5 there still is no GeForce 900 Maxwell accelerated graphics card support as the Nouveau developers are still waiting on NVIDIA to release the necessary signed firmware images. Beyond that, the GTX 750 Maxwell Nouveau support is very limited and not recommended for use. With Fermi still lacking re-clocking on Nouveau, the Kepler graphics cards are really the only good bet right now for those wanting to use a modern GPU with the Nouveau driver.
For the Linux 4.4 vs. Linux 4.5 vs. NVIDIA 352.63 proprietary driver comparison, the GeForce GTX 650, GTX 680, GTX 760, and GTX 780 Ti were used for benchmarking. All of the OpenGL benchmarks on these graphics cards were tested via the open-source Phoronix Test Suite benchmarking software. When testing the Nouveau driver on Linux 4.4 and 4.5, the Mesa 11.2-devel Git code from the Padoka PPA was used along with the xf86-video-nouveau 1.0.12 DDX. The default DRI2 was used since DRI3 is still having issues with Nouveau.