I ran my GTP at Autobahn (South Track) a few years back at a SCCA event with some friends. Being this was my first time ever driving on a course like this (did parking lot AutoX before) and only having attended the drivers meeting in the morning, it was an experience.
Overall I did ok, I only had brake fade once (scary for a second or so). Fourth time out, I went a little too fast into one of the corners and my brain did not want me to take me foot off the brake. I started in on the turn and the back end started coming out quickly (again...scary for a few seconds.)
I believe the best I ran after 5 runs was a 2:13 or 2:23. I believe the highest I would let myself go was 105 on the back straight.
The suspension and braking mods I had at the time were GMPP/Koni struts and springs, GMPP sways, poly bushings and endlinks, f-body front calipers, Powerslot rotors, Hawk pads and summer tires.
I would recommend minimally upgrading your brakes and track tires.
Also, I've had a tranny cooler on my GTP since 2000 and on my SSEi since 2006. No leaks, no problems.