OKG,
With that weight load you're not going to get 15 MPH. I just upgraded my 9.5, on a 14' tinner, to a 25 HP. With the above load that you stated, I got 10-11 MPH with the 9.5. With the 25 HP I'm easily doing 21-22 MPH, and that's not WOT. At WOT for some reason the handling gets a bit...