Being an older version it most likely had the higher resistance speed control. There were sooo many variotions on that dual motor platform made from 1985-2002. The earlier trucks had the worst controls but some even from the late 90's came with a high resistance controller which made them worse not only from the factory but much worse with a hot motor. I have never in 100s of cars had one lose power because its own speed control, however i have seen many of similar platforms run better than one or the other model because it came with a better speed control than the other. The hi roller probably got one of the hi resistance controllers and if you swapped it out with another stock controller not including an identical replacement you would probably see a difference.
By the way, if i convert my thor to hobby ill sell you my speed control/servo for cheap if you dont convert yours