Ok here are the pics as promised...
As you can see the US spec air box is small with little surface area and the air has to travel through another restrictive medium (charcoal filter) before getting to the engine. In theory this has to rob the engine of power.
Here is the Euro spec airbox (looks better IMO)
Inside the airbox (K&N E-2022 should be the same filter BMW uses in the performance kit) I was going to measure the surface area of each style of filter but just eye balling the euro filter it appears to have more surface area, thus allowing more air in and the air doesn't have to travel through a restrictive charcoal filter.
Overall this mod can be done for 400 bucks (new parts from dealer and K&N filter online) and it was well worth it IMO.