And speaking of M3s - last week I went to Austin to pick up the one I bought for my sister. I put a few hundred miles on it and am happy to report that it's EXCELLENT. Not sure what I was expecting but I am really, really happy with it.
And AP on M3 is great. I was kind of confused at first because it seemed more stable than my MS, but later I realized that being smaller it has more visible margin and probably just feels more comfortable because of that. It probably moves around about the same amount. The MS is wide enough that judging distance on the passenger side can be challenging but the M3 seems to be less so. In any case I had no problems using it in and around Austin.
I remember a similar experience going from an A6 to an A3 with their L2 ADAS. The A3 just felt more comforting, and it was a combination of the car being smaller and being more nimble so you felt like it had more of an ability to stop on a dime if you needed to.
The S/X are giant cars, and really wide too. It really does not feel like it has much margin to either side for error, which makes AP's job even harder.