To be honest, drop in filters are one of those things that simply won't give you any appreciable gains. For me, any small gains there may be are offset by the risk mentioned of less than ideal filtering performance.
I wasn't aware folks have done oil tests to compare contaminants with one of those K&N's on but the results bman spoke of wouldn't surprise me. IMO, there is a reason some of those drop in filters have "better flow". ......they simply don't filter as well.
With the stock setup, I'd stick with OEM or similar. .....personally. If I were looking at a CAI depending on the application, setup, etc. it might be another story.
@Kean LOL, I actually did do a before and after test with K&N with Blackstone Labs oil sample!!! And yes, it showed higher silicon content when running with the K&N. You can bet I tossed the K&N after that!
I swapped my K&N air filters out of all my cars, including the Subi, after seeing the Project Farm video a few months ago. Went back to OEM or paper media equivalent ASAP. The amount of particulates that made it past the K&N filter media was pretty astonishing.
As others noted, the minuscule performance gain, if any at all, to me isn't worth the additional risks and potential damage to the engine.