I found a set of momo corse wheels in the size listed above in a 4x100 lug with a 42 mm offset. I don't know the center-bore yet but has anyone had any luck fitting something this wide in an r56 and still being functional with minimal rubbing at full lock? I see that there are JCW wheels in this size so I would assume they would.

 

If you are running OEM suspension you will be fine

18x7.5 et42 sticks out 12mm more than OEM and is no closer to the inner suspension parts.

For that wheel size you can run the OEM 205/40-18 tire size.
You can also run 215/40-18 which is a little taller.

see
http://www.tirerack.com/tires/tires....=yes&tab=Specs

 

stock ET is 54mm?

 

No, stock et for 2007 MCS with 17x7" wheel is 48mm but the wheels you mentioned are 7.5".

So 6mm from the wider wheel and 6mm for the difference in offsets gives 12mm.