I'm not sure if the motor is your problem. If you run that model without a decoder (on DC) does it run noticeable slower than other DC locomotives you own? Maybe it is just geared slower.
All N scale models (from all manufacturers) are designed for max speed at 12 VDC. But there is also a wide range of what the manufacturer considers a maximum speed. Again, that depends on several factors.
The motor could be the culprit, but it is not because of its current draw. The decoder should be able to provide all the current the motor needs at whatever voltage you have the DCC booster set to. That is why you should compare the speed of that model to others you have using straight DC and no decoder. You will most likely see the difference in speed, even in DC.
Even motors from different manufacturers all run at different rpms at some given voltage. However I have never seen anybody post analysis of various manufacturers motors (stating specs such as current consumption or rpms at various voltages). Actually, that would come in very handy for re-motoring.
I suppose that if you find a motor which will run at higher rpms than the original motor, you could re-motor your model and it will run faster. Re-motoring will be easier than re-gearing. Using a Kato motor might do the trick - Kato models seem to be speed-demons. But I don't know if that is due to gearing or to the higher rpms of the motor.