I tend to agree with the less pitch more diameter, and here comes the BUT, I would use a tach and be careful not to exceed the manufacturer's max rpm numbers. Less pitch will cause rpm to go up however, there is a trade off. You will notice a drop in top speed of the aircraft and gain in verticle performance up to a point. A glow motor is designed to turn approx. 12000 rpm, don't expect your gasser to match that. Where a change of prop pitch on a glow motor may give half again the rpm change (8000 up to 12000) a gasser will probably not gain as much due to the internal mass of the engine.

Also be careful in just accepting HP numbers that you read in the ads. Many times the only way to acheive those kind of numbers is with tuned pipes etc. Also, HP can, and I feel is often, published based on calculation rather than actual measurement. There is also a difference between BHP (brake horsepower) and HP (also commonly refered to as indicated horsepower) which is oftened based on theoretical calculation rather than actual measurement. Some ads rate theirs in BHP other give it in HP. Do a search on the net for good info regarding horsepower values.