Bought a new Pipercross panel filter a couple of days ago, after fitting noticed the turbo lag between gears has reduced by a noticable amount, and it makes a nice 'sucky' sound too when I clog it after changing gear!
Even though the car has a full dealer service history, the filter doesn't appear to have been changed at all since the car was new- 25K miles ago! Needless to say, it was pretty grubby. Don't know about you guys, but I've always changed air filters about every 8K miles (standard service interval)? Think I've just found a flaw in the 'long life service' scheme ;-)