Just some random thoughts and opinions. Not sure if anything here can help you:
I have a fully built E90---gutted, caged, full aero, OS Giken 1.5-way diff, BBK, JRZ, race slicks, etc. I always run -2.2º camber in the rear. My rear tire wear has always been perfectly even and my rear grip has always felt great. I run 1/8" toe-in on each side in the rear----total 1/4" toe-in in the rear. This has resulted in very stable braking for many years on this platform. And I am quite hard on the brakes since I am supercharged, like to brake late and use high grip slicks.
Since you are having problems, I would recommend you give this a try. The extra toe-in should give a bit more stability in braking, and slightly less camber should put more tire surface on the track. It's a low commitment change. If the alignment doesn't help by itself, you also might want to look at your suspension settings.
What was the theory for going with 1150lbs springs in the rear? In my opinion, such a stiff spring is not necessary in a gutted/light car and makes it edgy and unfriendly on berms. (Others would argue with me.) And most of your weight loss came from the rear of the car. My gutted E90 weighs 3212 lbs (no driver no fuel). I'm at 900lbs springs in the rear. I've experimented with several rates in the rear over the years. 1000lbs was more jittery/edgy than I desired. If you had stock suspension on the car before the race build, you have seriously upped your spring rates while significantly lightening the car---sort of a double whammy.
I don't know your build. Do you have aero?
For what it's worth, the owner of Cobalt Friction brake pads has always recommended a higher torque brake pad in the front and a lower torque pad in the rear on both my streetable M3 and my track dedicated M3. This advice has always worked out for me.
In the end, I believe the weakest point on an E9x M3 is the OEM single piston brake calipers. With a fully built car, I believe you can justify the cost of a decent BBK just from the fact that BBK pads are much larger, thicker and last longer. My friend with OEM calipers and Endless pads was burning up pads in less than two days......yikes. I'm a fan of AP Racing. Incredible brakes. I own two sets on two cars---the older CP5555 kit on my street M3 and the current Essex Radi-Cal kit on the track car. I was at a NASA event getting a check ride a week ago in my street M3 with the older AP kit. The instructor was like "woah woah!" as we went into the corner on a late brake. The car slowed 100% in control. After we got through the corner completely in control, he was exclaimed, "Nice!!!" hahahaha Can't beat a good set of brakes.
(Of course, I've been arm-chair-quarterbacking here, but I'm just trying to put ideas in your head.)