im a bit skeptical about these air filters. i a k&n fitted to my micra, and it made the acceleration slower, thats mainly because they filter the air better, which obviously creates more resistance. the good thing about them though, is like i said they do filter the air better, which should prolong the life of the engine. as has already been mentioned, they are a lifetime filter so should pay for themselves after a few years.
if you fancy really changing the filter, heres something im hopefully going to do next month. take the old air filter casing off just leaving the MAF. then attach a cone air filter straight to that, either by cutting the flange off the MAF or by fabbing up adapter plate. this should increase engine pickup because the increase surface area on the filter. itll also sound really nice to: i had one fitted on my vitara when i changed the carb, gave a nice roar when accelerating. if i do fit one next month, ill post some pictures up.