Superrationality isn't a substitute for heroic responsibility, it's a complement. Heroic responsibility is the ability to <i>really ask</i> the question, "Should I break the rules in a radical effort to change the world?" Superrationality is the tool that will allow you to usually get the correct, negative answer.

ETA: When Harry first articulates the concept of heroic responsibility, it's conspicuously missing superrationality. I think that's an instance of the character not being the author. But I think it's later suggested that McGonagall could also use some heroic responsibility, and this clearly does not mean that she should be trying to take over the world.