""What would be a good prop for this motor for just speed? ""

well, as I said I'm using a 3W 20x8,, and at over 15 pounds my plane moves along at a pretty good clip, so probably an 18x10, 18x11 or a 17x12, 17x13,,

now, if you went with a carbon prop you may gain a few hundred RPMs, even more..

Jim