The pulse width has nothing to do with velocity or acceleration - it's a matter of ensuring the minimum pulse width is enough for your motor drivers to reliably detect the pulse, and it can be, sometimes very significantly, affected by your BOB. I've seen several, usually CNC4PC brand, BOBs that SERIOUSLY mangle the signal. What you really need to do is get an oscillocscope and see what the step pulses look like AT the motor driver pins. You'll likely find they look VERY different from what they look like at the BOB output pins. In any case, at 25MHz kernel speed, a setting of 6 has absolutely no down-side whatsoever.
Regards,
Ray L.