It depends on many factors, but chances are good you will not notice microstutter with 2 580s. Kepler cards and the latest graphics drivers suffer less from microstutter than in the past (or reputedly crossfire). The biggest factor is how much you perceive it...people are different in their tolerance for framerate. Even with one gpu you get big framerate differences: do you notice this and does it bother you? If not, you probably won't notice it with 2 580s.
Thanks, I'm kind of sensitive to those little things, though, so I think I'm better off buying a 680.