I fitted K&N to mine, obviously difficult to judge the difference but it (may) have made it slightly louder and a little bit freer-revving (I'm pretty sure about the first but the latter may be pschological).
Bear in mind though that you need to clean the airbox out regularly and make sure you service the filters exactly as K&N describe.
There's a school of thought that says the K&N can cause more/faster damage to the sensitive MAF sensor, not experienced this myself but ymmv