Which tires did you have on? Although I expect you are a lot better driver than I am, your experience with the brakes and handling on your stock suspension were the same as my first (and only) track day. My track was shorter, being only 1.76 km, with two short straights. The longest straight allowed me to get to around 100 mph before braking, and the torque out of the turns leading into the straights was such the the M cars could not overtake before the turn at the end of the straight. I had to wave them on to pass, which was only fair since they were better than me on the twisties. At the time I had the non-sports suspension with the only change being Bilstein B6 dampers. I also found I needed to be judicious in the application of throttle coming out of turns, even with full nannies, because the rear end did want to break free. I do think my OEM RFTs got a bit slippery after a couple of dry sessions though. I say that because the last two sessions were wet, and it was quite slippery for me. But the rear would want to break free even in the dry. I have not been back to the track yet to see how the LSD and reworked suspension and tires will change things. Drag time is surprising. Most guys report that traction is the problem and the difficulty of getting a good launch. Maybe your tires had got a bit slippery too after the track work?
Mods-Etc: Quaife LSD w/VAC finned diff cover, M3 Rear SF Bushings, M3 sways & OEM Susp Front/Rear Cmpnt Kits ('cept strut brace), GC Caster/Camber Plates, AC Schnitzer M3 Sport Springs, JRZ RS1 dampers, JBD 100%, Goodrich SS brake lines, non-stggrd APEX ARC-8 18" wheels with Nitto Motivo tires (winter OEM 17" wheels and Blizzak LM-25).