Sammy and Osterizer:

Correctly geared, it will run cooler and you will be able to keep your head speed up when you increase pitch quickly. Over geared motors heat up quickly.

I ran with a 10T on my 3550 motor and it would shoot up without even slowing the motor, but the head speed was juat about 2500 with the 10T and I wanted to be closer to the 2700 range. Basically 11T.