This is why:

Nice craftsmanship but I hate to say it but our stock filter is huge and not a restriction. Even with it dirty, I reach at most 1-2 in h2o on my restriction gauge. Max 30 psi and all the way to redline (max mass flow is at high rpm and high boost). We have more of an issue with exhaust manifold pressure as tuning goes up.
If you take that k+n and unwrap the filter element and measure surface area vs the stock panel filter it will be close. My money is on the stock filter having more surface area. This would be a great mod on the 335d where they have an overly restrictive filter.

The piping is not a restriction at all. It's bigger all over cross section area wise than what the turbo inlet takes in.