Mike
Ive had a K&N element in my Mk1 172 for about 10000 miles now - I took it out recently and replaced it with a new paper filter and it initially seemed better. However, I noticed that within a month or so the engine didnt seem to pull as cleanly in the mid / high rev range as it had, so I replaced it with the (still rather dirty) K&N and bingo! Happier at around 4000rpm+. The car just feels slightly more response and generally a little more free-revving at high rpm. Thats it really and Im sure theres little/no actual performance difference...
Id honestly say that theres not a huge performace difference between a clean paper filter and an aftermarket replacement, but the K&N certainly seems to retain its flow even when far dirtier. Thats just my feelings anyway, dont know if anyone else agrees, and Im also not sure if its enough of a difference to warrant paying out the extra lolly on insurance!
Matt
