I posted a thread earlier this year with a similar dilemma when going from 18" to 19" rims:
https://f30.bimmerpost.com/forums/sh...1704497&page=2
Summary:
I went with a 245/40 19 tire from the stock 225/50 18. The rims were the 18x8" ET34 BMW 400M and the 19x8.5" ET33 BMW 351M. No spacers, no rubbing with this setup. I'd really like to see if it's possible to run a 255 width tire in front on the 19" rims, but I don't want to be the guinea pig to buy tires.
I'm very happy with this setup, ride is much better than the stock RFTs. I run 35 psi front/ 38 rear.