K&N cone on mine, cleaned and oiled twice since Nov 07, no issues. Only oiled from the outside, and left overnight to 'dry' before re -fit and use.
I'm not sure if mine has been chipped, but with a standard box and new standard filter, it always felt like I'd selected a gear or two higher !!
The difference was so obvious, that my Mother ( whom often takes it on long runs for me ) asked if the handbrake was binding .
I've used K&N panels / cones on at least half of my previous cars ( inc diesel turbo, petrol turbo ), never had an issue, MAF or otherwise.
Takes 10 minutes to remove box and fit the 57i kit Rob