Here's a link with almost all the motor set-up possibilities: http://rctouring.eu/electric_motor_set_up.htm
''By cutting your commutator the diameter will get smaller. This has a result of more timing. It's like every millimeter that the comm is cutten, is a degree more timing. Keep this in mind when you run 1 armature on a long race weekend with much runs. Your last runs, after cutting much of your commutator, will have more top speed and punch but will also less runtime! Calculate and experiment with giving less timing to prevent losing the race by a battery dumping failure''
The brush hardness could be also a factor, check the link.