I agree my 9.9 will get my 25' to 5.5 mph and my buddy has a 17' alum boat with the same motor and gets the same mph even though my boat weights 3x more.The extra weight of a 15 is not worth what you will gain the other factor is "hull speed" the faster you go the boat will try to get on plane (which will never happen) and will create more drag in effect negating any gains from the extra hp.