OP may have meant brushes as I can't think of a single thing on the starter motor that would require shimming - sounds DODGY.
Below is a pick of brush set.
The above is an after photo as I soaked it in vinegar and water for a week.
Below is the before shot (was submerged in water with battery connected)
Could be he shimmed/adjusted the contacts between the motor and the solenoid? As they do get arced up a bit. Let me check and see if I have any pictures on my computer.
If you decide to do the starter motor mod looks at these contacts and the ones in the starter and use the best. But I can see and remember the slop in the holes the contacts bolt in and the ability to adjust the contact surface even rotational.
Oh .... I thought his starter was junk because it was noisy. Why in the heck would the dealer "shim up" his starter in the first place? Sounds like a shade tree mechanic to me.
If it was power flow what the heck is the shims for?
Geez, I was about to pay $459 for a used one on flea bay. I'll get onto these suggestions. Thanks you all. Never did understand what they meant by "shimming it", but it made it work better a while..
I'm not sure its been a while but all I did was take my cables to a local shop told them I want thicker gauge and about 3 inches longer because of the added stiffness.