It was wet on the way home, so I only really opened it up twice. I didn't notice any wandering in the front tires. If anything, it seemed like the back end tried to get loose for a split second (like a RWD car).
I drove both my car and a M3P with the 20" wheels. Acceleration seemed the same on both. Handling may be a bit sharper on the 20" wheels. Ride was nice and smooth on both.
My car's window sticker lists it as a Model 3 Performance, then shows the 18" wheels in the options list. It has the red line under the dual motor badge and on the ID screen, as well as track mode under driving options.