I love hearing the sound of the turbo spooling up and down with k&n filter, maybe me still being a bit childish coming from a Scooby but also very handy to audibly guage driving economy - thats my grown up answer anyhow. Have had K&N on rav for over a year now, no issues and on the auris 180 too, both fitted with Kingos boxes.
I guess it depends how short his short urban trips actually are? If it's only 2 miles each way then he'd be lucky to get more than mid 40's. Also depends on the roads he has to go on etc etc. One persons town is different to anothers.