[slicer]

255/30/20 on the front is the best size option but it will likely rub the back of the fender liner slightly when you are at full lock. The rubbing will not be a big deal. You will rub with anything wider than a 285 in the rear with that offset. I would absolutely run a 285/30/20 in the rear. An added bonus is that you will get a taller sidewall over the 305/25/20 that you were considering.

Overall 20's are not well suited for an M3 and are a compromise in every way. Looks, performance, price, comfort, strength, weight, tire options, etc. I wouldn't recommend anything bigger than 19" on this car.

OEM offsets are a good guideline but very few aftermarket wheels are built to OEM offsets. Most people want more aggressive widths and offsets to fill out the fenders. You are also running an aftermarket wheel in different widths than OEM.