Not sure what you mean.. I would say 2 microseconds is the sensible default since the latest and the greatest in terms of stepper drivers is a TI DRV8825 (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slvsa73e&fileType=pdf) and it requires a 2 microsecond pulse (see spec sheet on link). However.. let's say EasyDriver comes out with a new board that has 64 microsteps --- pulsing at 2 microseconds is going to be very slow (especially in the case of a 400 step motor).

Another option would be to change the function to FirmataStepper::stepMotor(byte step_num, byte direction, byte pulse_width) <---  I guess that's what you mean?