I think "looks can be deceiving". You have TWO 50mm props driven by counter rotating 550 motors with 3:1 reduction (either gear or belt, doesn't matter) so you're talking about a LOT of thrust. Would 60mm props be better? Maybe, but one thing for sure they would definitely be over working the motors (I know, you're going to water cool them, still you'll be working the motors a lot harder with 60mm props). Five blade 50mm or 45mm could make a difference without killing the motors.

Right now things 'look' pretty good. You're going to neede to finish up the boat with something like 1 oz. fiberglass on the hull, seal the inside, get your deck with push knees installed, bitts and any other deck hardware you want, a light weight superstructure (build it out of styrene or sintra), get all the gear inside and hooked up, THEN you will know for sure about the props. Whew!

Bob