The 18x9 ET30 will be quite aggressive in the rear. Then again, 'aggressive' is sort of a relative term. We generally recommend 9.5 ET45 with a 245/35/18 tire on the rear of the 1 Series. That fitment doesn't require any modification. Your 9 ET30 would stick out 9mm more.
This is what our recommended 19x9.5 ET45 V702 rear looks like on the back of the 1er.
