90deg C is 194 deg F!
The GTB is designed to themal at 200deg F, so I bet it's thermalling and losing a deg degrees by to time you temp gun it.
The reason for you thermaling is a 4.5 and Lipo is absolutly madness (no offense) in a buggy. The only way you could make that drivable is by deliberatley running the wrong gearing which will make it run hot. A correctly geared Lipo 4.5 will be like a cruise missile in a straight line.
In a 2wd I run a 7.5R on a tighter track like York on Nimh. I tried Lipo there for the first time tinight and the 8.5 I was running was actually a little bit faster than I wanted
