It's always worth playing with a design to try to make it better. That said, I think adding 3 or 4 inches to the middle will have little, if any, effect in the directions you are looking. By adding to the widest part of the boat that sits deepest in the water you will move more of the displacement to the center of the boat. This will tend to lift the ends out of the water, minimizing the "keel effect" which enhances tracking. Lifting the ends will also shorten the waterline length, which will reduce the hull speed. As an added penalty, you'll be adding wetted surface.