I've read on at least one occasion where someone tested std filter (in good condition) vs K&N and basically saw no changes in times - and by the same token there are lots of comments from people saying that their car feels more responsive and "breathes easier" etc. with a K&N fitted.
I have a K&N filter myself - always have and always will - but not necessarily for outright performance reasons - I never did notice any change as such once it was fitted. My reasons are that the K&N is less likely to suffer as time passes than a std filter, and the K&N is better in the long run for cost - to keep a good std filter in your airbox you'd easily be putting 5 new ones in during the same period a single K&N will last (probably even before it needs it's first clean!).
In short - performance improvement? - not sure - insurance against loss of performance? - definitely. It is simply the premium solution for engine filtration.
IT'LL GO OR IT'LL BLOW!!!