One thing you have to keep in mind is, that the 9.9 and 15 are essentially the same motor. Converting from 9.9 to 15 by changing the carburetor is not going to make the boat jump up on plane any faster or provide anymore performance in the lower rpm band. The only difference will be on the top end. A 9.9 will achieve about 10 hp around 5000 rpm's and 15 hp at around 6500 rpms. The bigger carburetor allows the engine to reach those higher rpm's therefore achieving up to 15 hp.
Let me give you an example. If you had a 14 foot aluminum boat with a 9.9, two big guys, trolling motor, batteries and more gear than they really need. Running at full throttle the boat reaches let say 16 mph's at a rpm of 3800. If you were to switch over to the 15 hp carburetor you wouldn't see any differences in the speed. The motor is not able to reach the top rpm's. You would need to either lose some weight or change the prop to a lower pitch and then the motor might be able to reach those higher rpm's. Now, if your boat is able to reach a speed where the rpm's are around 5000 than the bigger carburetor would help to achieve a little more top end which would translate into more speed.
So if you read the article it talked about how one would use their motor. If putting around a lake and trolling is all you do then a 9.9 would be a better choice as it has a better/smother idle. Where as the 15 hp is used more for high speed runs.