My friend and I were playing some RU and his Slaking, in the end, was banded. I had my HitMonchan out and he went for the CC. He, before that, used Sucker Punch, therefore locking him into that. Next turn, after his Truant, I outsped with the Mach Punch and killed him when Sucker Punch would have killed me. Here is the replay: http://pokemonshowdown.com/replay/uususpecttest4835047
Sucker Punch is only +1 priority. If the Hitmonchan was faster than Slaking, it would've outsped with Mach Punch.