It is not dangerous. In the summer, I am running 8" wheels with 205-tires!
I think BMW prefers to equip the M135i with wider tires in the rear to get more power down in the summer. In snow and slush, you get more grip by running narrower tires, so it is a bit risky to use wide rear tires in the winter. 225 all around makes sense, and it doesn't matter if the wheels are 7.5" or 8".