For a particular motor there is no way to compare the performance that you get if you run it straight out of the package versus what you get if you break the motor in on the bench. Before the brushes are fully seated the contact area is very small so you are applying more pressure per square inch to the commutator
. I used to recondition armatures for a number of people and it was easy to spot the ones that did not get a bench break-in. There was discoloration to the commutator
that was not removed when it was cut and there was also damage to the edge of the slots. When the motor is not broken in properly the crystal structure of the copper is altered to something that is much harder and does not conduct electricity as well. When I first started truing armatures I was suprised at how fast carbide cutters would wear out. I had to switch to a diamond cutter.