Will games run better on SteamOS?
Possibly, possibly not. Windows has a lot of overhead, and bloat, but much of that bloat is size rather than performance, and while there is noticeably performance bloat... when you're talking about a quad core or more, it's not that huge a deal, though latency between EVERY call to the gpu, which can be thousands upon thousands every second do add up.
In general it's possible to beat WIndows performance, that doesn't automatically mean SteamOS will, because MS can improve windows performance, and DX performance, and they just don't bother doing more than they need to. There is potential for someone to make a slimline OS that does help performance wise, doesn't mean they can't screw it up and make it worse, doesn't mean they'll hire the right people, and doesn't mean their focus is entirely in beating Windows in performance, can doesn't mean will.
Ultimately till 6 months from now + at least a half dozen games released that work on both OS's, we can't really tell anything. If HL3 comes out and is 20% faster on SteamOS, I wouldn't be surprised, but that doesn't mean, I dunno, Watchdogs won't come out and be 20% faster on Windows. Valve would likely put more time into optimising for SteamOS and less time optimising for Windows, another game might be the reverse. SteamOS might end up being insanely buggy and almost unusable, far far too early to tell.
