M5Ranger

I believe I know what you are pursuing. You can get competition compounds for the rear in 18".
I had HRE custom make me a pair of C20's in 18x11". Can now put on drag radials. I've been happy with the performance of the BFG's. The Nittos were not as impressive to me.
Presently am awaiting the release of a new tire by Mickey Thompson for this application. Run the 305/35/18, this will give you a rolling diameter of 26.40" and stock is 26.85" so that's pretty close and no problems.

Good luck
Ranger

ps. Sorry if I discussed more than you wanted.

__________________
2006 M5 Alpine White/Silverstone
Upgrades:
Engine: SS headers,Evosport power pulley,Active Autowerke
ECU,ASR custom air intakes,Dinan throttle bodies, ASR Engineering NOS kit, Evosport 3.91 diff.
Exhaust: ASR Engineering custom exhaust.
Suspension: H & R sport spings,Dinan front/rear CF strut braces.
Wheels: HRE 590R, (F) 20x9 (R) 20x10.5 w/ Michelin PS 2 255/35
& 285/30
Exterior: Vorsteiner CF vented hood, Vorsteiner CF front spoiler,
ACS rear deck lid spoiler, front kidney grill & side gills in
CF.
Brakes: Brembo GT kit, (F) 6 piston monobloc calipers w/ 16"
rotors & (R) 4 piston monbloc calipers w/ 15" rotors.
Interior: ACS titanium CF trim.
Dyno resultstraight motor 460 rwhp, on nitrous 581 rwhp/461 lb/ft rw torque

2007 GT3 speed yellow
2007 GT3RS orange
2007 RS4 imola yellow