[petek]

Dredging up very old memories from my school days, I think the reason for difference in fwd vs. rev speed is due to optimization of the angular position of the commutator brushes vs the permanent magnet (stator) field, designed to maximize torque. Think of it as being the DC motor equivalent of cam timing in internal combustion engines.

You can build a permanent magnet DC motor with equal torque in both directions, by "centering" the brushes, but you sacrifice peak torque in doing so. Which would suggest that if you wanted to modify the drill motors' brush holders, you could adjust for equal speed/voltage in both directions.