Will I notice any loss in ride, handling, or acceleration changing to the new Michelin AS3s in a slightly larger size( 205/40-18 Dunlop Sport Maxx to Michelin 215/40-18 non RFs)? My car is a '19 JCW with the stock sports suspension. This is my fourth MINI and the first that I have not minded the ride with the stock RFs. My problem is that the Dunlops are Summer tires and in a few months I will be going on a long trip and need to have an all season tire for some of the colder parts of the trip. I don't need snow tires. In addition, would it be wise to just stick with the stock size,
(205/40-18)?

 

I think swapping from the stock 205's to 215's is the right move. First they fill out the wheel like I think MINI would have intended. I run 215/40/18 Michelin Pilot Sort AS3+ on my '17 JCW and there is NO rubbing, loss of handling in ANY respect. I think you will be pleasantly surprised with your choice. Tires are grippy and quiet on the road.

 

I think the difference going from RF to non-RF is going to trump everything else. That alone will have the new rubbers feeling worlds better.

 

I finally bit the bullet and bought a set of Michelin AS3+ in 205/40x18. I was thinking about buying 17" wheels and runflats, but the Michelins are so good and my experience with them has been when you get a puncture, the tire will lose pressure very slowly. My wife just got a flat in her Audi S6 ( Pirellis ). She must have picked up the screw close to home because there was no warning from her tire pressure warning system. Luckily, I was just checking my tire pressures in our cars and her right rear was dead flat. After that experience, no more Pirellis for me.
Anyway, with Michelins you truly get your money's worth.

 

I replaced the Dunlop Sport Maxx tires which, runflats , rode decently. Comparing the two tires, the Michelins weigh the same as the Dunlop RFs. But, the tread on the Michelins is 1.7 iches wider ( same advertised size). I think ( a personal opinion ) that the Michelins are marginally better in every aspect ( ride, handling in both wet and dry, and I can use them in below freezing conditions).
Another thing in my experience with tires, Michelins seem to hold air longer when punctured thansome other brands, i.e. Pirelli.
They're not "r" level track tires, but for me, daily driver they're hard to beat and they literally last forever( 40-50,000 miles is easy).