I know what you're describing with the sagging.
I've seen GT 150's so smooth they look like somebody spent good time on them with a long block ... mine, rippled all over the place. It bugs me, especially after a wax when the boat is shiny and all the reflections are distorted because of it.
I'm not sure, though, that the cause is what you described. My "sags" and "ripples" are maybe 1/8" at most ... I guess a good body shop with some body filler could smooth it all out ...