really is dependent on the particular motor, and the installation. motors are designed to sink/bleed a certain amount of heat over time used. if you up the cell count, you also up the potential for heat. if the motor is over designed, or if you have a really good install with a lot of air flowing over the motor, you can get away with 1 or 2 cells extra, but it will still generally shorten the lifespan of the motor.
i seem to be remembering, that bob is a member here, so you might be able to pm him through the RCU system. uses some form of his name for a handle.