Is SteamOS Any Faster Than Ubuntu 15.10 Linux?

[Nobu]

The Steam client has X11 riddled through it like cancer. Games are a different story.

Even that being the case (which it appears so), you could run steam through xwayland under a lightweight wayland compositor, and have the games run directly ontop of wayland with little overhead, since steam could unload it's window while the game is running or stop its render thread.

[lvlark]

the point of steamos is not the performance, is the easy to use, ppl simply install the game and run it

Yes - but they can't really afford any significant gaps in performance.

The point of steamos is to give Valve a distribution channel where they don't have to fork over a percentage of their revenue to another company.

Certainly not the only point of SteamOS - there's more ways in which it could represent added value to Valve. But I honestly doubt this is the place to enter into a detailed discussion about that.