"Tech support told me to cut the prop until it turns the desired rpm on the ground"

What is the desired RPM on the ground? How is this number determinined?

2 stroke engines produce their max HP and torque at a certain RPM. Not to say that this can't be increased, by a tuned pipe or more nitro. I'm not sure if you should prop an engine for max torque or HP, my gut feeling is HP. Once this is done, going for another 1000 rpm wouldn't gain much in the air. I think this is why your not seeing increases in the air, your engines are maxed out as it is.

There are many factors to consider here. At some point increasing RPM makes the prop less efficient, the trailing blade travels through the proceeding blades vortex, causing cavitation. A singleblade prop is more efficient than a 2 blade prop because of this.