AriuSen,
Looks like you had a fun and exciting time!
Your car was obviously a handful, and I think much of that was attributable to rear springs that were too stiff compared to the fronts. You were getting a
lot of oversteer, even with throttle-neutral cornering--that is, not induced by strong acceleration.
I don't know what spring rates are best for a Z4M, but I'd guess that the 150 lb differential between front and rear was considerably too much. The back end was having to do more than its fair share of total roll resistance, which was loading up the outside rear tire a lot more than the outside front.
I also noticed, especially in your earlier laps, that you were getting a lot of oversteer when you let the clutch out after downshifting as you entered the turns. Absent a heel-and-toe downshift, to match the engine rev's with the axle speed, the driveline has to pull the engine speed up to match, and that creates quite a braking effect. Enough that it was throwing the weight forward, lightening the rear end, and forcing the tail of your car out. You did well to catch it!
In friction circle terms, the deceleration force on the rear tires caused by the engine, plus the cornering force from entry into the turn, added up to quite a bit more than the tires could handle. Thus the rear tire breakaway.
Anyway, it all looked like a lot of fun. Compared to your fastest lap, however, when you completed your downshifts before turning in, your earlier laps with all the tail-wagging really killed your lap times. Heel-and-toe downshifting requires some contortion, and sensitive application of both the brake and the accelerator with just your right foot, but I think you'd find it enormously helpful in smoothing out your laps and increasing speed. It might not be quite as much fun, however!
I suspect you already know all of this, but I thought I'd provide my 2 cents, just in case. Good luck with the new springs, and have a great time.
Rick F.