I have -2.0 on the rears and currently have -2.2 on the fronts.
Used -2.6 originally on the front, but while great on the track, had some inside wear problems on my street tires with long highway miles.
-2.2 a good compromise for both ( or get off my butt and actually use my adjustable chamber plates...)
2011 e92 M3 Monte Carlo Blue ESS 535 - gone but not forgotten
2015 cayman GTS - new toy
2006 SLK 55 AMG sold