I stole this photo off "Boosted Falcon" and based my calculations off it,.
It's running 18x10 +17 up front and 18x10 +15 on the rear.
I used the offset calculator on 1010TIRES.COM and came up with the following.
I want to run 18x9.5 Rota P45R2's and they're available as +12 and +20 and I'm aiming for good fitment.
Based off the calculator if I run the +20's on the front I would get 3mm more clearance on the inside and would retract 9mm from the outside compared to the 10" +17. If I had +20 I'd get 11mm more on the inside and retract 1mm from the outter.
For the rear I'd defintely go for the +12's since they would retract only 3mm from the outside and give me 9mm more on the inner, this is going off the wheels in this photo! Should look okay I think!
Couldn't get the uploader to work so here is a link!
http://img.photobucket.com/albums/v204/oneredED/f2a459f7.jpg