With numbers like that, something is wrong other than the prop. On my '85 204 Fisherman, I could spin a 15 x 17 3-blade AL wheel to 5600 - 5800 depending on load with my '02 Merc. 150.
Where's your anti-ventilation plate in relation to the hull bottom? Are you sure you're getting full throttle (the cable isn't binding somewhere or hitting something)?
