Thanks Alan. I'm not exactly a car guy before this but I imagine going from 20" to 18" with the lowered suspension might require a change in that as well, right? I asked my Tesla dealership about swapping the PS4S for Winter tires on the same Rims and they said it was $2000 (it's $4000 with the rims on their site) and I almost gagged. I think if I switched to 18" tires i'd stay there forever and try and sell the 20's.
Either way thanks for the response and I'll keep that all in mind and report back later. Jealous of your weather over there
Do be aware that above 35-40 degrees or so, MXM4s will have inferior handling and braking compared to the PS4S, and may still not be great in winter. 60-0mph stopping distance is 125ft for the MXM4 and about 105ft for the PS4S (265 width) (still need a datapoint for the stock width here...will get some decent data at some point soon).
It's going to be cheapest to find an appropriate 18" wheel. There are discussions on this forum about exactly what fits the AWD & P3D+, and be aware it is different than what fits the AWD due to the reduced thickness of the rotor on the P3D+. The 20" wheels are terribly impractical, and sounds like you can't hit anything with them, or they'll bend. The 20" wheel itself provides minimum handling benefit (the tire compound dominates that). You should not need to make any suspension changes, though you do have to be sure that the wheel you choose will clear (again, there are extensive discussion on this elsewhere).
I guess you have a few options:
1) Cheapest is to stay with what you have and avoid driving in freezing conditions (PS4S is not safe in freezing conditions). This is ok if you're not taking any road trips and you aren't worried about your charging costs (which will be 10-15% higher than what they could be with optimal wheels).
2) Probably the best and most convenient is to get a second set of wheels that fit (18" for reduced tire cost), and put the MXM4s on them. However, you're not going to have awesome winter performance. But, you could run these year round. And sell the stock wheels & tires if you want. You might end up net positive if your wheels are in good shape. Sadly it probably won't look as good. You don't have to worry about aeros (though they will help a little), but maybe someone will come out with aftermarket ones at some point.
3) You could get another set of wheels with winter tires on them. The efficiency would probably be better than the PS4S but hard to say. You could run these in winter & the stock wheel/tires in summer.
4) Another good option is to get two sets of new 18" wheels and run high performance summer on one and winters (or an all-season winter) on the other. Then sell the stock wheels/tires. For road trips you might want a wheel with low rolling resistance tires like the MXM4s, though. It all depends on what is important to you - performance, winter grip, and range. These have to be traded off and the more wheels you have the more flexibility you have! But 18" is the way to go to keep tire cost down.
Personally I've done nothing, and am trying not to hit any potholes. But at some point I might autocross and for that I'll want 18" wheels for cost & durability (Qualcomm Stadium parking lot can have some potholes open up during autocross). But I'd also like a road trip car...and the autocross wheels will not work for that due to the sticky tires.