I would agree that your speed, in good water and light weight load conditions should be much more. Anything in the 35-40mph on gps would be a range I’d expect with your boat and motor. Just to be on the safe side, have you checked on each of the following to make sure they are not contributing to the problem:
1) Are your three toons clean - no build up, film, growth deposits, etc...?
2) Is your engine mounted properly in the right hole/height - is your cavitation plate at a good spot when under way and at WOT?
Regardless of proper prop, both of the above would slow you down considerably if off.
My assumption is your prop is off, but I’d check the above two items as well.
For prop ideas, I will leave it to others with the 150 Yamaha’s to impart good advice. We have a number of them on the forum, and I know they will have ideas for dialing that in, assuming you are not already using a good prop for your set up.
Good luck!