Re: 15 horse upgrade
Change those jets and all you will do is have a lousy running 15, Ducks. That added 33% of HP is achieved by what you can't see.
You will be much better off fine tuning the balance, rigging and propping. First you will need a tachometer. Without it you will not be able to measure any gains.
Do you have PT&T? Trim is the first place to go for best speed. Then weight distribution in the boat. Next would be engine height and finally prop selection.
Fixed trim can be a problem. Fix it for best hole shot and your WOT will suffer, fix it for best WOT and you may not be able to even get on plane.
Set the engine as high as you can get it without prop ventilation on turns. That typically will be the anti ventilation plate 1"-2" higher than the bottom of your boat.
Trim and height can be made adjustable with aftermarket "jack" plates.
Select a prop that will allow you to reach, but not exceed, the top of the operating rpm range with a normal load.
All of these "adjustments" are trial and error, so don't expect pat answers. The best answers are unique to your boat and no other.
Good luck.
