Picture it this way... if the engine is running, the exhaust turbine is turning which means the compressor is also turning. If that is turning, it is moving air. This is the key, the air is ALREADY moving when it comes into the engine. It doesn't have to go into boost to affect efficiency. Anything that will increase velocity even slightly in the manifold can give better fuel distribution cylinder to cylinder which will make the engine more efficient which could equal better mileage.

Kevin