I have had good result with K&N filter on my Ford Explorer, my gas mileage increased 1-2 mpg after I replaced the paper filter with the K&N, at that time, the paper filter was still new, I had less than 5000 km on it.
K&N did make a noticeable difference on my Ford, after the replacement, the engine started to run very rough, I had to reset the ECM computer to let it calibrate itself, then engine smoothed out.
Haven't tried it on my 400E yet.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.