23mm poke from oem is equivalent to putting a 23mm spacer. just get a ruler and see where the line for a little under an inch is from the outer edge of where your rear lip is now.
if you look at the 513m on the car in the link below, the outer edge of his rear lip is about 3mm more than where your rear lip will be.
you could probably even get away with 18x9+30 up front, tbh. little poke there as well and similar to the 513m on the front of the car in the link
http://f30.bimmerpost.com/forums/sho....php?t=1427977