As said, if it's a '82 or later, the exhaust tuner between the 9.9 and 15 is different. The carb is different too but a 9.9 with a 15 carb and no other mods is only about 11 hp. The tuner is worth some power...and RPM....and obviously speed.
The stock prop doesn't like to be run shallow, so lifting the engine on the transom much more than about 3/4" above the bottom isn't going to play well with the stock prop. At least thats the way mine is. I switched to a Solas 4 blade 9x10 aluminum. Much smoother and stays on plane at a lower RPM/speed which is more important than fastest top speed.
I wouldn't mess with a hydraulic trim if it were mine. You start adding weight. Might as well get another 9.9 and run dual engines...LOL. The 9.9/15 are really lightweight and I suggest using that to your advantage. You can play with the trim pin and get it real close to what your needs are.
Jackplate not really needed on a little 15 HP. IF your transom will allow it, a transom lift adapter can be made cheaply which will raise the engine however much you want it to be. I used a wooden 2x2 under the motor, clamped it, tried it out and it seems to be just about right. Then I bolted the motor to the transom. It is solid. Cheaper than buying a mini-jacker. also lighter weight. My 1432 is a heavy boat (plywood decking on front and all plywood floors) and runs around 21 mph @ 5800 GPS and a tinytach. Could stand a bit less pitch (9 vs the current 10) but it works for what I do with it. The 15 likes lots of RPM BUT you don't have a 15...you have a 9.9 with a "little" more extra power. The tuner will help a bunch and then you can turn the RPM up to about 6200 or thereabouts and still make power where the 9.9 with 15 carb signs off at about 5800-ish.