I have had a K&N filter on loads of cars over the years and HAD one on my 520d until last week.
I changed for a standard filter and the results were really good. From the get go there is more torque and smother acceleration, so I had this explained to me the other day and see if you think it seems right.
BMW make a diesel motor with a standard air filter the size at which is flow rated to the power and pull of the flow of air required for the optimum smooth running of the car.
Then they create an inter cooler again which is sized to match the rate of flow of gas through it. Its job is to cool the flow of gas so the motor can burn it again at the optimum temp, thus more power and efficiency.
Right if we put a bigger or less restrictive filter in the air box it will allow more air in. The ecu will compensate for this a bit by fueling more but the inter cooler is still the same size. So the bigger volume of gas passing by the inter cooler can not be cooled at the same rate as a standard flow of gas so is warmer and less efficient so the motor makes less power.
I have found this with changing back to a standard filter. Having this explained to me made me try it, and the motto ere is with modern diesel motors standard filters seem to be best. This may not apply to petrol engines. Does this sound right to you.