Have you tried the Mercury Prop Selector ? It has a limited number of other engines in there and you can usually get a decent approximation of your own set up to get results. Just put in the nearest details available to your boat and engine package.
Guessing a few of your figures and using a Rib as a bench mark (No Sibs on selector) it does suggest a a higher pitch prop than your are currently running.
Prop Selector | Mercury Marine