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Another point of reference from the track on PS2s. I just got back from WGI. I ran into a guy I know who was driving his E36 M3. He had some worn out PS2s, which he seemed to have driven to the track on (he was swapping track wheels on). The PS2s were on or close to the wear bars, so ~2/32-3/32" tread. We started talking about their grip although he wasn't driving on them on the track yesterday. He said he used them on the track several years ago and tossed them aside (probably because they were worn). Then he bought a 911S and a 997 GT3, so it sounded like he hasn't driven the M3 much--at least not on the worn PS2s. Then he said he recently drove on them at LRP with the worn out tread, and turned faster laps on them than he did years ago. When I asked about traction, he said they were fine, and whatever driving skill he picked up over that time made up for whatever loss of traction that might have occurred.
So, I still don't really get how your F/R traction balance was altered so drastically because the fronts were slightly less worn (since he has not reported a drastic loss of grip, and I haven't experienced it myself with my own worn out/destroyed PS2s). I still think of the following 2 potential explanations:
1. Tread compound somehow changes significantly beyond 2/32", but we don't have any evidence and it's not clear why a manufacturer would do this. I can only assume it would cost more to build tread with varying compound.
2. You are just getting used to how the car behaves when pushed in a track environment. You mentioned you have a good sense of the oversteer behavior of the car from street driving, but I don't see how you can really unless you are driving it recklessly. I corner nowhere near any kind of traction limit at 60mph+ on the street even when I'm having fun, and that is pretty much what you have to do to learn about the handling of the car at the limit. I guess you'll find out if this is the case or not when you go back to the track on newer and more evenly worn (F/R) tires.
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