Both of my cars are running 18x8 wheels with 225/40 non-runflat tires on the fronts. My wife and I have no problems driving either car, and the 128i was upgraded from 17" runflats. Both have sport/M-sport suspension. The 128 handles much better with the 18s.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2011 128i M-sport Cabrio 6MT/PE