My experience, after owning MINIs for ten years, is the tires wear pretty evenly. I basically do the scenario you described, move the fronts to the back when worn and replace all tires when the former-back-now-front tires get worn out. Only difference is I alternate the same process with snow tires, so it takes me longer to go through a set of tires. Before the SE I'd go about two seasons on my all-seasons (which I typically run in the summer) and three or four on my snow tires. I do expect my summer performance tires to wear out faster than all-seasons.