FWIC, your rpm is way to low if you can only hit 5k!! Its not a diesel LOL
My buddies f150s really shined once he propped them to rev 6k when the boat had a light load.
My experience has been good with 4 blade props, they hold better at lower speeds but I have lost a couple mph top end but I have only used mercurys Vensura and revolutions.
The revolution is with out doubt the best I have used, and what I would suggest you try in a 15p
I would say you could easily step down 4", one thing to remember your most efficient rpm will change with the different pitch so its not going to be as bad as you might think.
It will be reving higher at any given mph but will be working less, if that makes sense? Load will kill it quicker than revs.
Phone around, some shops will take your GOLD Visa and let you try a few props on the agreement that you will purchase one.
Jac also correct about hight but I would prop right before messing with the hight if you can see your ventilation plate when up on step.