I'd expect there to be a sizeable fraction of people who are atheists merely as a form of contrarianism.

I don't think that's the case. I do think there are a good many people who are naturally contrarian, and use their atheism as a platform. There are also people who become atheists after having been mistreated in a religion, and they're angry.

I'm willing to bet a modest amount that going from religious to atheist has little or no effect on how much time a person spends on arguing about religion, especially in the short run.