Sorry, I don't have a link, but I vague remember someone on another forum putting the 19" 1M wheels on a 1 series coupe.
I think there were some additional work that needed to be done, they didn't just fit.
You should be able to get 18" reps though?
As always, the lower the profile, the harsher the ride. The 1 series already has a pretty unfriendly firmness, especially with sportspack.