With Mesa 17.0, OpenGL 4.3 is still exposed by NVC0 even though it implements all the OpenGL 4.5 extensions, it doesn't yet pass the GL CTS. But at least with Mesa 17.0, NVIDIA Maxwell support goes from OpenGL 4.1 to 4.3. There are also other new features to Mesa 17.0.
I ran some Mesa 13.0 vs. 17.0 vs. 17.1-devel tests with a GeForce GTX 680, GTX 780 Ti, and GTX 980 Ti. Of course, couldn't test any GTX 1050/1060/1070/1080 Pascal hardware as there's no accelerated Nouveau support until NVIDIA releases the needed signed firmware images... Only the GTX 980 Ti was tested of the Maxwell family since there is no re-clocking support there yet with Linux 4.10. And then the NVIDIA Kepler support with the GTX 680 and GTX 780 Ti remain best-off on Nouveau since at least there is manual re-clocking support and with 4.10 there is the boost support. For this testing, the GTX 680 and GTX 780 Ti were re-clocked to the 0f performance state and the NvBoost value was set to 2.
Thus the GTX 600/700 Kepler cards when re-clocked remain faster than the GTX 900 Maxwell series. Maxwell re-clocking unfortunately isn't coming to Linux 4.11, but we can always hope it will for Linux 4.12... Along with Pascal consumer card support.
The GTX 680 and GTX 780 Ti don't see much of a performance change when going from Mesa 13.0 to 17.0+, unfortunately. The GeForce GTX 980 Ti though is noticeably faster with Mesa 17.0.
While crippled due to the lack of re-clocking, the NVIDIA Maxwell performance is faster with Mesa 17.0 due to the the instruction pipelining work that landed for 17.0 and was accomplished by Samuel Pitoiset.
All Maxwell hardware on Mesa 17.0 should be much faster thanks to this work by Samuel. But still it's a messy situation without re-clocking.
Team Fortress 2 is playable with this open-source NVIDIA driver.
Those wanting to explore more of these OpenGL Mesa 17.0 Nouveau test results can swing by this OpenBenchmarking.org result file.