The flaps, at least in the VW application, don't really to much in the way of changing runner length. They reside at the end of the intake manifold, facing the cylinder head intake ports.
Their primary function is to optimize cold start and idle mixture (increased swirl effect/turbulence) for economy & emissions. They have been proven to affect power at the high end, and kits are available to delete them now.
On Audi/VW V8 applications they have multi-stage intake manifolds which do alter the intake length, allowing for longer or shorter runners as desired.