I bought a new pair of OEM rear 8.5" wheels for my ZHP and am running 255/35-18 KDW2 tires all around. It works perfectly with no spacers needed. It helps reduce understeer significantly. The car is setup for D-stock autocross, so it has the camber pins tapped out up front and am running a larger front bar (stock class doesn't allow rear sway bar changes). With the added negative camber and larger front bar coupled with the 8.5/255 all around setup for the street, the car handles perfectly. No rubbing, plenty of clearance, also looks great.
For autox, I run 41mm 8" front and 44mm 8.5" rear wheels (requires stock widths) with either 265/35-18 or 245/40-18 all around. Currently using 245/40 BFG R1s.
2011.5 E90 M3 sedan 6MT
2001 E39 M5 LeMans Blue/Silverstone
2007 E91 328i Silver/gray
2006 E90 330i Electric Red/Black 6MT