In addition to the recent post about Turbo Magazine getting a 4hp gain with the K&N panel filter in a Maxima, I read an article in SCC about their turbocharged Impreza 2.5RS. With the stock airbox, they replaced the stock filter with a K&N panel filter and instantly got an extra 2psi of boost, which means more power. So, apparently, those things are worth a few horsepower, contrary to popular belief. When I had mine in, I definitely noticed better throttle response over stock, and it was probably making more power, too.