Raising the exhaust means filing away .75-1.0 mm from the front edge of the exhaust port(closest to the head). This lets the exhaust port open a litle earlier and gives it more duration, leting the engine breathe better. Will wind out a little more. Smooth the ports out with emory cloth, and be sure to remove any burrs.
I have done the same thing with my #11 high torque cylinder, shaved head, derestricted stock exhaust,15 bing, 17x41 gearing. On a good day with no wind it will do @44mph.