In addition to water dipping, you can try some Trinity bushing buster on the bushings to accelerate break in. I also have used Brasso on the comm (after water dip break in is done) to polish the comm. We've seen 500-1,000 rpm increase, depending on the motor. Just put a drop or two and run for 1-2 minutes with a 4-cell batt.
With Mabuchis and Johnsons, sometimes it's the luck of the draw on which motor you happen to buy. You can find significant rpm differences between motors.