The speed penalty depends upon how fast MESS already is, or will be. For bsnes, it wouldn't even be noticeable because everything else is so slow already :P

It's just that SNES users are quite picky about having their NTSC or HQ2x filters. It's easy to say you don't care, but you can't very well test every last game ever made yourself. You'll need to get some hardcore fans to help you with that.