I don't know anything about 550 motors. A TC is a much lighter and lower drag car. 4.5 can blow the tires off of the things. 4.5 is what most of the pros use for mod TC racing.
Plus it's all about gearing and speedo set-up too. If it too slow, go up a couple teeth on the pinion.
Sean. Certified speed crazed mowron.
Team Shepherd USA