Yamaha sends a lot of their motors to the dealers without a prop. Oftentimes, it is up to the dealer to select a prop to put on the motor and marry it with the boat. More times than not, many of them don't seem to get it right.
I have some experience with that exact motor as my dad has it on his boat and his boat, in my opinion, was NOT up to par. I have the same hull in my boat and I noticed a huge difference.
If memory serves me, his boat came with a 13x15 and had similar issues. Don't quote me, but I beleive I went to an SS 13.75x17 on his boat. Now I am right in the RPM range while increaseing top end. The holeshot was also greatly improved as the motor was underpropped and seemed to "Spin out". Keep in mind that his was an aluminum one from the factory and I switched it out with SS.
With Pitch, the lower the pitch, the greater increase in RPM (about 200RPM per 1 inch of pit). Raise the pitch and you will lower the RPM.
With Diameter, you increase the diameter on a slower boat and decrease it on a faster boat. Look in your book for RPM range but, if memory serves me 5800 at WOT is what you should shoot for. 6000 would proably be ideal. So, based on what you said, I would go UP to a 17P and probably up to a 13.75 diameter. Raising up to 17 should decrease you by around 400rpm, increasing the diameter should acount for some of the speed you lost when you lowered the RPM.
Hope that helps.
I have OCD "Obsessive Crappie Disorder"