Interesting, at about the 14:45 mark, Lars (the VP of vehicle engineering at Tesla) discusses how the pirelli summer tires were specifically designed for the M3P, and that switching to non-Tesla, regular summer tires can potentially impact range by 10% (he quotes that he's guessing), but he sounded more certain when he cited a loss of 5-7% in lateral grip.
So for those of us looking to downsize in wheel and tire size, our only option (at least for now) would be to hopefully access the tires that come standard on MIC M3P since those come with 19s.