As other's have said being forced into 19's isn't the best situation to be able to run sticky tires - especially the Hoosiers that you like. Square has its benefits - namely being able to get longer life out of the tires. The downside is the amount of potential grip you are not utilizing in the rear. I personally feel square on this platform is too much of a compromise in times. Coming from the Porsche world with more of the weight pushed to the rear you might find that you prefer that extra tire in your M2C with the S55.
ZM2 I believe runs 19 on the track - might get with him about it. Apex also put out a good thread about 19s -
Apex Wheels 19" Fitment Thread . If you find that the stock brakes are not sufficient & move to a BBK (Essex AP Racing
) then you could get down to 18 and have a lot more options. I run 275 front & 295 rear Hoosier R7's - the handling is excellent.
Suspension - I love the Ohlins R&T on my M2. I also love the JRZ 3-ways on my E92 M3. The M2 being a dual duty car I would absolutely choose the Ohlins again if I had to do it over. I prefer the ride vs oem on the street - and the car handles great on the track. They offer different rates of springs as well - I went with their standard recommendation and am happy with them. Highly recommend you look into these if you consider upgrading the suspension. Excellent upgrade for a dual duty car.
Camber plates - keep in mind that the plates will have to match up with the dampers you are running. If you decide to upgrade the dampers need to have the plates to match. If you wait on the suspension for awhile the plates to match the oem dampers would probably still be worth for the tire savings alone - plus the better handling characteristics.
With the M2 (and comp more so) you really have to look at the above like one system. Certainly true with all cars but the M2C seems to have a few more restraints than most. Wheels need to clear the bigger brakes, camber plates fit the dampers, if you add a coilover setup like Ohlins then you have to consider fitment conflicts with the spring perch, limited front tire in front. Just a lot going on there that helps to have straight to help avoid multiple purchases of the same items.
Are you DCT or 6MT? If DCT then I think you will find a DCT cooler to be necessary. Check out do88 - it has been stellar in my track E92 M3 that I could overheat even on street tires. 6MT - would recommend removing the clutch delay valve & upgrading the shifter (CAE in particular).
You might find cooling overall to be an issue. If so would recommend looking at the rest of the do88 line of coolers.
Enjoy!