I have 20mm rear spacers and am trying to find a local person willing to let me test fit to see if the 18's then have enough room to clear the rear calipers of the P+. So far, nobody is willing to meet up. I guess I need to be in California.
As far as what is affecting range, I would like to see what efficiency the RWD gets with the 20" wheels. Currently, we're seeing anecdotal evidence that the P+ is taking at least a 25% range hit when driven similarly to a RWD. The EPA testing shows there to be a 7% difference between them. The Michelin PS tires are a special compound that is low resistance for its grip, so it isn't a regular Michelin PS tire like the MXM4 is. Tire width is the same, so it isn't that.
In city driving, rotational weight of the 20" 31lbs wheel should definitely be an impact compared to the 20lbs 18" wheel. That plus difficulty to gauge gentle acceleration from a 450hp car and 270hp car are difficult to determine. I'll just assume that it is a big impact for city driving where I'm in town and charging at home every night. My range doesn't matter there.
At highway speed, aerodynamics should affect it more than the weight because it is at constant speed. However, constant speed on highway is really where we see the range difference. The 20" and 19" should have near identical aerodynamics and definitely better than the 18" w/o aero cover.
Then, some will argue that it doesn't matter how lightweight the wheel is because a 20" will always be worse than an 18" because the barrel carries more weight and is 1" further away from the center. While there is some logic to that, the tire is still there and in some cases, the narrow sidewall tire makes up for the weight difference of the wheel. It's hard to believe that the wheel barrel being 10" from center vs 9" is going to decrease efficiency by 15-25%. No logical sense.
Some say that if you have a wheel with lower offset to be closer to flush, it is going to completely destroy the aerodynamics. They say that there is more surface area of the tire that air is hitting along the curve of the front bumper. I have 15mm spacers on my P, and the efficiency is no worse now than before I put them on. If the tire extended past the wheel well (poke) and disturbed airflow, there could be a difference.
So what is best? Get lightweight 18x8.5 wheels, low resistance 235 wide tires, and create an aero cap.
Another aid: Lower the car. You will improve aerodynamics by lowering the car a bit. How much will it help? IDK, but it helps not hurts aero efficiency.