I'm running a 18x9.5 Advan ET25 upfront with 275 35R18 with no fitment issues.
I suspect the extra half inch of rim width is causing the wheel to stick out just a little more which is causing the rubbing. If this is the 'proper' fitment for the M3, maybe it is because in track setup, there is more negative camber which allows for the extra track width. Just a thought.
It was great meeting you last week. Beautiful car.
2009 E92 M3 Le Mans Blau on Schwartz. DCT. RAC RG63, KW V3, Eisenmann Race, AP Racing, Vorsteiner GTS5 Front Bumper, Vorsteiner GTS-V Rear Diffuser.