Speed: 6.5 knots with 15HP outboard, 9 knots with 40HP
A little extra speed's expensive, isn't it?
Not only is the cost of a 40HP outboard a lot more expensive than a 15HP outboard, it's going to use a whole lot more fuel in the process and its upkeep will be costlier as well.
Of course, everyone wants to go just that little bit faster... Don't they?
Offhand, it seems to me when the gain seems meager compared to the cost of attaining it, you might want to take a long hard look at your priorities.