Agree with NOS. Hard to believe you'd need a shorter prop.
Are you sure your throttle is actually opening all the way? Might have to pull the cowl and see what moves when you open the throttle, then take the boat out and manually actuate it while running (ideally with a couple of people, so you can do the motor stuff while someone else drives.)
Do you have a tachometer on board? Does the motor spin up past 5000 at all?
The recommended WOT range for your motor is 5000-6000 rpm. Ideally you want to be in the upper half of that range. That would mean (rule of thumb - probably not that accurate) a prop shorter by about 3 inches (9" pitch.) That sounds ridiculous for a boat that size.
Make sure your tubes are inflated properly, and the boat isn't loaded down with a bunch of extraneous gear that you don't need. Check the hull and make sure it's clean and smooth. Ditto the lower unit. Make sure you remove the trailer from the boat before speed testing (yeah, just kidding.)
White Noise
