It sounds like you could use a wider line spacing on your handle. When I set up the line spacing on a handle my initial setting is the spacing that will move the controls their full travel with the airplane sitting on the ground. With the way I move my hand an airplane with a 4" bellcrank the spacing ends up at about 4 1/4, with the down line spacing more than the up line. If you know you are getting full mechanical throw and the plane still feels mushy, try some tailweight. That is what I did on my last plane. I keep a dedicated handle for the main stunt plane I fly, because a lot of the trimming is done on the handle end.
Mike Greb