I have a 90 hp Honda on our fairly light 24' boat. It's currently using a 13p prop, turning it right up to max rpm for good (best possible) pulling performance. If it were not for the pulling performance I was looking for, I would likely go to a 15p prop to get the rpms down a little.
Without being able to run at wide open throttle, and being able to see what rpm the engine is turning at that point, coming up with the correct prop for your application will be difficult/impossible. Are you running into this problem with the engine trimmed all the way down?
"Top end" and "pontoon boat" are mutually exclusive terms. Pontoons are not fast, though 25 mph or so with your combo is not out of reason. It's about what ours does.