It's more like getting a lot done and breaking things for everyone else. Imagine your boss has a terribly, horrendously stupid way he wants things done. Overperformance would be going over his head to his boss to try to change the awful policy. It might be the right thing to do, but you're only going to piss off your boss in the process. Michael is saying it's better to do what the boss asks in that case, but only do the minimal amount of work necessary, instead of going off on a crusade that will only cause trouble for yourself and provides no benefit.