Well it depends on the engine. If its carburetted, then manifold suction is crucial, so improving the seal for high suction will limit the vacuum leak which the valve is designed to attempt to seal in the first place. It is supposed to only start opening and allowing the controlled leak once load is applied and/or throttle is opened but the standard design is a little primative.
EFI on the other hand is so well controlled a little vacuum leaking isnt neccesarily going to cause enough trouble for this mod to be worth it.