I would, personally, probably make the decoys a bit bigger, but the same speed as the real missiles. Since the real ones will also be in slow carriers, have the decoys release a bit before the warheads.
In my case, that wouldn't really work. The entire carrier complex is set up around carrying Size 1 missiles, so I'd have to rework all of that before I could make them bigger. Actually, I think there are problems with the whole idea, as missiles don't release precisely at the boundary. They release instead at the first increment inside the boundary, so you'd probably have to set the decoys off far enough forward to negate the protective effects from beams.