The NVIDIA vs. Open-Source Nouveau Linux Driver Benchmarks For Summer 2018
Due to the lack of re-clocking support for the GTX 980 Ti and GTX 1080 Ti graphics cards, it's no surprise the GTX 600/700 series hardware is multiple times faster in their re-clocked form. But even with re-clocking, BioShock Infinite is one of the games where the Nouveau driver still leaves a lot to be desired.
Dota 2 is another game that's quite rough on the Nouveau NVC0 performance for Kepler and Maxwell1 even when re-clocked.
While the aging Portal game does run at playable frame-rates on Nouveau and even manages to perform modestly against the proprietary driver for the Kepler/Maxwell1 hardware. The GTX 780 Ti on this fully open-source driver could achieve over 70% of the performance of this proprietary driver.
Nouveau on Kepler was working out well for Tomb Raider where the high-end GTX 600/700 GPUs could run with playable frame-rates at 1080p... But of course the newer GPUs remained crippled due to re-clocking.