Even though it may seem 'counter-intuitive', many times I have seen that going to a smaller prop improves performance significantly. I'm sure you know Dan, but for those that may not, prop size is like the gearing on a car. Too tall a gear ratio, and the motor lugs, overheats, and speed suffers. (It's an inverse relationship. 2.56:1 is 'taller' than 4.11:1) Going to a smaller prop is like going to a lower ratio gear in a car. Of course, it's possible to go to a too small prop as well. And go nowhere.
There is a sweet spot that will be the ideal situation of speed, run time, motor temps. And it really helps if you have a box full of different props to test. That's an expensive proposition when using bronze or brass props. Thankfully, there are inexpensive plastic props that can be found, used for testing, and then a suitable bronze prop substituted for it when the sweet spot is found.