I think Performance Ford did a review on this, it could have been in the RS Turbo Special - will have a look later.

If i remember right, overall it didn't make a huge amount of difference - the std airbox did pull in less warm air but am sure the car produced slightly more power with the K&N cone filter on - as i say it's been a while since i read the article but am bored at work so

I would like my std airbox back on next time the car is set up, as don't like the noise from the K&N and it cheesegrated my hand too many times lately when i've been messing with the car