I have experience with both K&N filters on my 1.4:
The inlay filter: doesn't feel much different from the stock filter. The only thing thats different is that you hear a 'swoosh' when you depress the gas pedal.
The open K&N 57i however is a different story... when starting the car, you hear the air flow through the filter. When you start driving, the feeling is better. I always had the impression that the 1.4 is very sensitive... only 2 options: you depress the gaspedal too little, the car stalls, or you depress it too much which results in a wheelspin. With the 57i, the car is easier to start, because of more feedback. The other effect is, when you depress the gas pedal, the 'swoosh' mentioned earlier is much louder
. When you drive normal, the car is as silent as usual (except for the sound of the air passing the filter), but when driving sportly, the sound is incredible! (especially accelerating from 110 km/h to 120km/h at full throttle)... going from a rumble at 2000rpm to a sound that makes you shiver down your spine at 4000rpm... very addicting
Both the filters have no real effect on the fuel economy, but you save money with every check-up, because the flters do not need to be replaced.
