I've thought the same thing about my 16.5 - fishing. I was looking in the 3.5hp range. But it comes down to what your pushing and what the motor has to over come. The Honda 2 will easily take you up to max rpm's so there is nothing really to overcome. To get more speed you'll need more rpm's or a change in the pitch of your prop - to my knowledge there are no props to swap with. The manufacturer would void the warranty if you did anyway. So what would you gain going to a 3.5 or a 5hp? Very little. The rpm range is the same and I believe your over-all speed would increase 1 or 2 mph? BUT, the noise... the weight of the motor... would be making the biggest difference, I believe - that may be your biggest issue to consider. I don't think you can go wrong with the Honda 2. Many folks use them on their 18's and 22's - that says a lot.
Enjoy -