I agree. In fact down aileron can cause tip stalling at a higher speed due to the increased effective angle of attack so it can have the opposite effect to that required.

Spoilerons or air brake set-up is more effective for slowing the landing speed. This is UP aileron. It will give effective wash-out at the tips to help prevent early stall conditions developing. This setup also can give added angle of attack while at the same time preventing tip stall. This gives a high frontal area of the the model to the direction of travel. Slows it down more for landing.

Try air brakes to give a fixed few degrees of up aileron when you throw the switch - works well on fast landing aerobatic models, so should be good for a 3D model.

Flapperons can be dangerous for landings...