Sorry, wish I could give you more insight. I will tell you that many on here run 255/275's. At the even yesterday, one of the instructors, who was driving a caymman, also has a z4m. He has tracked his z4m a few times. I told him I had 235/265's ready, and his comment was, "next time go with the 245/275 set up, they'll fit." Ultimately, he thought that was a better set up for autox. The other big issue is the camber both front and rear. I understand pulling the pins will only get you so far. to get 1.5 - 2+ you'll need to go with washers or plates. You could probably also get more if you dropped the car a bit, but that's getting pricy.
Lastly, from what I've read here, a strut bar will greatly help turn in. This is probably going to be my first addition. You can pick one up from tischer for about 300.
I'm gonna take me car out to a highly respected tire/race shop here in the next few weeks to have the new tires installed and start working on a camber setup. Once I do, I'll report back with their recommendations at that time and let you know what transpired.
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