I'm running Win8 right now and I'm running all my Steam games like normal. Same FPS. Same everything. No special permissions required.
Nobody is going to use Metro UI exclusively on the PC. Most everybody will use the desktop and treat Metro like a full screen start button.
The only real complaint I could maybe see is that by default the Metro UI has a bunch of crap apps on it from MS that point to MS stores. Most gamers will dump those in the trash and set up their own to replace them.