Actually, a leading edge cut will advance the timing and make the motor spin faster to the sacrafice of efficiency and torque.
A trailing edge cut will retard the timing and make it more torqy and possibly help the efficiency.
I've found that a hole in the positive brush will help the efficiency drastically and push the rpm up a bit. Power stays the same with a shift to the higher end. Torque drops.
Cutting the corners off of the trailing edge of the negative brush should reduce the arcing and burn, though I don't have enough experience to comment on the torque and power impact.
I need more torque and wanted to confirm the trailing edge cut. I've never tried it before. I'll probably try it tonight with some 4499s so I don't waste my Putnams on it. If it works, I'll do it to my putnams.