When running Unigine Heaven 4.0 on SteamOS at 1920 x 1080 on SteamOS Alchemist Beta, only the TITAN and GTX 780 Ti graphics cards were pushing above 60 FPS. Heaven, of course, is a very demanding benchmark and not too representative of most modern Linux titles offered on Steam. In terms of why these benchmarks here weren't benchmarked through the Steam download software itself, read Running Benchmarks On Other New Linux Games and Finally, Team Fortress 2 Benchmarks On Phoronix for details. It covers why we can't use Metro: Last Light for Linux benchmarking and other challenges faced by using Steam in automated and reproducible tests.
Sanctuary is Unigine's least demanding test case. Again, the results coming out soon will put the data in much better perspective about the performance of SteamOS. These results today are just some reference results for those anxious to see SteamOS benchmarks or want to run their own performance comparison against my numbers using the Phoronix Test Suite.
For SteamOS it really is just a matter of consistently delivering above a 60 FPS experience (or whatever your display's refresh rate may be) and with minimal latency and input lag, since vsync is enabled by default. The graphics cards falling short of this are the GeForce GTX 550 Ti and GeForce GTX 650 from this Core i7 Haswell system.