Calls for rigor are unproductive, I think. The unfortunate reality is that an experiment with the rigor you want would be prohibitively expensive. The big differences in productivity I suspect are going to be in larger projects over longer periods of time. You also can't figure anything out from small sample sizes, so you would need to take four large teams, split them randomly into two groups of two, and have each time solve the same large problem over a long period of time.

Unfortunately, since we can't afford the rigor, we have to make do with anecdotes and less powerful studies.