Mine was doing the same thing with a SP 4.5T MMM geared at 8.0 with Stock 1 firmware with/without boost + turbo. It did not have any heating issue with a Reedy 5.5T.
Try another motor.
My son runs a SP MMM V3.0 4.5T on an older SP GT 2.0 Pro esc on a small indoor carpet track, all timing and boost turned off. FDR 7,66. Tamiya TRF418.
Esc loaded with mod firmware. SP states somewhere not to use stock firmware on a mod motor.
40x40mm WTF fan attached, and it can run for 15 min. straight and only gets about 55 degrees. Can run until LVC kicks in (5600mah).