Keep in mind that this was a Z3 and not a Z4, also it was a 2.5 and not an M.
My rear suspension was blown out at the end of five years and I decided to replace it with KWs. When I did so I was already running the max camber in the rear with the OEM set up due to wear and just general sag. I lowered the car maybe 3 to 3 and a half inches in the rear providing it with around 3.1-3.2 degrees of camber. It did have a bad toe in problem on the rear right which I also corrected, I ended up running about .25 toe on the left rear and about .4 on the right rear. This was done with RCABs (knowing it wouldn't last forever).
Regardless even at stock camber of right around two degrees and a full degree of toe I was still able to run about 10-12k miles on my tires.
I do not have an answer to what you're looking for as far as maxing out the rear camber, but I feel like I've since seen threads and builds that incorporate much more than -2* of camber. I would look at Rated M, he's generally a wealth of modded information.
Sorry for jacking, but I figure this will make things more clear.
Also on softer tires I didn't notice that big a difference in tire life. At least in "general quality sport tires" not slicks or anything crazy. The wear maybe progressed a little faster, but that may have been due to driving as well. Expect maybe 2-4 thousand less out of "performance tires"