PMDC motor: how much is voltage reduced if the resistance produced by commutation system is eliminated?
You haven't allowed for the back EMF in the voltage calculation.
There are 3 sources of voltage drop opposing the supplied voltage :


I (5.75A) * R (0.25 ohms) in the winding, = 1.43V.
I (5.75A) * R (0.85 ohms) in the commutator/brush, = 4.88V.
Back EMF which is speed dependent, and currently 14.8 - 4.88 - 1.43 = 8.49V


Eliminating the commutator losses (and maintaining the same speed and torque) reduces the voltage requirement by 4.88V, giving 9.92V.