OK I still beleive power can be improved but that can come later try this to get a prop recomendation. http://www.ballisticprops.com/MIWheel/propit/main.htm
This is a online prop calculator and if you give them the correct info they will give you the right prop for the job.
It is not the fastest service in the world may take them a week or so to get back with a answer but you will get one. Most of my posts have centered on the power issue as a whole it takes a certain amount of power to move a boat thru water at speed. In rereading your replys to me and others I noticed that in the 2 props you have tried the rpms stayed the same only the speed changed this again makes me wonder about the engine cam and possably the igniction system as a curiousity what does the engine free rev to in neutral? If by some strange deal the engine has been limited in rpm due to a limiter or by malfunction the you may be able to go to a larger pitch prop and gain more speed. I say this because usually a minor prop change has more effect on the full throttle rpm than the boat speed and you adjust prop size to get enough load on the engine to keep rpms to a reasonable level. Simply put a smaller prop on most setups raises engine rpm but boat speed stays about the same just has faster acceleration and runs higher rpm at top speed. A higher pitch prop normally does the oppisite boat is slower to accellerate and engine labors at a lower rpm to make speed. A friend of mine had a master craft ski boat that had a rev limiter to allow them to use a lower pitch prop to get out of the hole and limited engine rpm to prevent over reving. I wish I had kept the 14.5x19 prop I had cause I would loan it to you to try but I threw it away a few years ago when I decided I was not very interested in boating at the time. go try the calculator I gave you and see what they say.