I added the K&N airfilter to my Focus yesterday morning.
i would say it gave a 1mpg increase in the readings from the mileage panel readings.
So I was impressed.
Then, looking at the stock air intake to the air filter box, it is an odd contraption, with a blind second end (for noise reduction, I had the same on my Mystique, it cancels the intake noise, and does nothing for performance)
I am wanting to let in more air too. A different intake lip from the stock air in at the front may be just as valuable. The air is coming in from next to the Ford front emblem, lower half. The stock intake part is pretty restricted, and actually narrows down, next to the blind ended taper.
And yeah, it makes a big differnce at low RPM if the intake has some restrictions, the engine just does not produce much vacuum to get the air flowing at low RPM. So less restriction means more air coming in, and more power made. The engine meters out the fuel totally based on the amount of air getting in.