We've been using 350fps in our indoor arena in Winnipeg for the past 2 or 3 years without a single serious injury. And this isnt seasoned airsofters playing, this is walk ins off the street ages 16 and up. Go there on a Friday or Saturday night and you could see a good half dozen girls per game too.
I dont know what would possess anyone to NOT wear a facemask in a CQB game, regardless of FPS. Thats just plain irresponsible, but that shouldnt be used as an arguement to set the max FPS to a point where not even a stock CA gun is legal.