This could be a stupid question......Are you running a motor sensor wire to your ESC? A bad (or missing) sensor wire could cause poor performance too.
I run a Hacker 21.5 with the timing cranked all the way up with gearing of 37/102 (5.51 FDR) in our USGT (Spec GT) class and the car isn't that much slower than the 17.5 cars running 4.0 FDR touring cars.
Motor temps do run in the 170 range with a motor fan, but then it's starting out at about 100+ degrees here in the desert, even racing at night.
Our GT class runs different gearing from most other GT classes for some reason, but with everyone running it it does make for some good racing. (We race on a relatively small track.)
You may want to re-gear your car so you can use 64 pitch gears, you can fine tune your FDR's a bit better. I tried to get away with using the stock spur gear but quickly saw the benefit to the 64 pitch gears and the wide range of possible ratios.