90% of the improvement comes from removing the charcoal filter and putting in a high flow air filter. The Dinan piping doesn’t really buy you much. Plug it up – you will be hard pressed to notice the difference.
BMW kept the closed box design for sound purposes, and to keep that choking charcoal filter. That is why cone intakes can make so much more power – they get rid of the charcoal filter. Cone type intakes also have a much, much different sound.
That charcoal filter just kills throttle response.