My mate went from 17's to 18's on his 3 door, and it spoiled the ride big time IMO.
He thought the same and went back to 17's.

Another mate has just took 18" Comps off his Saph. Drove $hite, but did look good.

Personally, Focus RS rims dont do a Saph any favours looks-wise anyway.
Each to their own tho........ just my opinion.