Sorry, this is wrong. It has nothing to do with RPM.

http://www.answers.com/topic/thrust-power
http://www.auf.asn.au/groundschool/propeller.html

The actual airspeed achieved is the important figure. The theoretical thrust of a prop at a certain airspeed is insignificant if the aircraft never reaches that speed. You could have a model that is draggy enough that a high pitch prop will cause it to fly slower. Not only that, take off performance would be miserable.

It all depends on how much drag the model exhibits, desired airspeed, and available power. You won't make a DR-1 fly any faster or more efficiently by putting a square prop on it, unless you can reduce the RPM to allow a larger diameter at the same HP input. This is what BLW is eluding to.