FYI, I have read that running a motor under heavy load with the brushes retarded can damage the comm. See:
http://ecomodder.com/forum/527-post189.html Obviously that'll depend on the degree... When I was playing with brush timing, I mistakenly set them retarded and could
hear the arcing on the comm before I even got out of the driveway. It did scorch the edges of the comm bars.
I predict you're going to notice a difference in performance (speed) with the brushes advanced.
The motor in the ForkenSwift was spinning the "wrong" way in its previous life as a hydraulic pump motor. After we reversed the field connections internally to reverse its rotation, the brush timing of course changed from 17 degrees advanced to 17 degrees retarded.
Result (no load): 1480 RPM @ 12 volts retarded, vs. 2275 RPM @ 12 volts advanced.