If your Missions are still in good shape, they should be a pretty decent match for the SL. The SL is such a great "middle of the road" board and is so versatile that you really can get away with damn near anything. I just personally don't like putting a super stiff binding on a softer flexing board; tends to make it very touch at higher speeds.
A stiffer binding makes things sketchy at high speed? Is this because you cannot "feel" the board the same way?