What are your other prop choices? It seems like you should be spinning that 17p 5000 and still have some throttle left. Just as a point of reference last season I ran a 21p prop and was hitting ~5100 at wot. That would get me to a consistent 53-54 mph on gps. My boat is a 24' Mach 1, not exteremely heavy or light. I had some old literature that gave a weight, I think it specd 4500 lbs with a 454. My 406 engine dyno'd 365 hp.

I've got an old 19p prop that you can borrow if you'd like for testing. It's pretty rough, but it might give you another point of reference. I've been keeping it around as an emergency spare, but I could live without it for a while.