I did all the searchs, found out a lot. but it seemed nobody had the rest of the story!

To make a Yamaha 9.9hp 2 stroke a 15 hp.
Install a 15 hp reed pack, it has 6 reed not 4. This is simple, it is right behind the carb.
That helps a lot but does not get you all the way there. the next thing you need to do is put a new 15 hp carb on it. the 15 hp carb has a 19mm air venturi and the 9.9 only has a 15mm, 20% more air to a 15. All the jets in the carb are the same, however the brass nut thing that holds the bowl on also has a bigger hole to allow more fuel into the bowel.
This cost about $260. and it will get you to 21mph vs 16.5mph in a 14 ft aluminum jon boat with 2 big men and gear. (ps you will also need to go to a 11 pitch prop to get the speed)

I have also heard that pre 82 mercs and evinrudes could be converted, but I never found out details.

I never fish 9.9 limit lakes and I am not trying to get one over on the laws. I just happen to have a 9.9 and would rather hop it up that get a new motor.

I have to admit a 15 hp out of 79lb machine is sure sweet.

With most 4 strokes you must change the cam so it can get much more complex and costly.