Too Much
I have a 1981 25' Mac. It has a 9.9Honda 4 stroke that ways about the sameas yours. It does a fine job, althoughif it weighed less, it would be alot easier to handle.The weight of your 15 hp. doesn't seemto be a problem, nor do I think torqueit would have, cause you trouble.But there is a mathematical formula thatsays a boat can only go so fast, unlessyou get it to plane.In other words, your mac is only going to go about 6 - 7 knots, whether its pushed by a 15 hp. or a 6 hp. motor.Personally, I think more power on demandis better though.