Ok, if the m motor was slow, l motor has to be very fast. I've noticed from the motor to the diff, its geard down from 12-20 tooth gears, from the diff to the hubs its geared down the same way, then its geared down a third time in the same way inside the hubs. Plus it's going to have more torque then the m motor because of the size of the set. So maybe (on another post somewhere, I'm not sure where.) I was right on the L motor being the high speed, high torque motor.
Edit: I noticed between the motor and diff, its geard down a second time, so its geared down 12-20 tooth gear reductions 4 times. This L motor HAS to be fast if it goes through that much gearing down