It will bolt right on, the 15hp carb has a larger venturi, which will give you more top speed and a bit faster holeshot
From past experience, the only real difference in the two motors is the length of the exhaust housing under the power head which is tuned for the 15hp. The general consensus is that you will likely end up with what amounts to about 13hp or so,
Your biggest gains will be in the higher RPM ranges and at idle they tend to run a bit rich but not so much it causes any issues unless your trolling at idle for long periods. You will hit max hp about 500 rpm higher overall then with the 9.9 carb