At a recent minor service, the service manager called to say the technician had taken out the air filters and, whilst not scheduled to be replaced, he felt they needed changing as they were dirty. I agreed. What a difference to the performance - point, squirt, go. I had noticed it was a bit off the pace, but thought I was just getting used to it. It's obvious really, with dirty filters not letting the engine breathe properly. Logic would dictate that the economy will benefit also - asthmatic engines aren't efficient. Anyway, my point is - check and change if required, separate to the official maintenance schedule - you'll be glad you did.