Well the banshee has no problems with getting air to the engine (stock) so a K&N wouldnt make a difference. The banshees stock pipes resctrict the flow of air out of the engine, so you can't really pull much more air in. (Without changing the pipes) When the pipes are changed, and you have more flow out of the engine then you need better flow into the engine (better filter) for power. I've heard K&N's do better than normal foam filters, in terms of performance. Wont be a noticable diference with low modded quad, but with alot of mods it may help quite a bit. This has just been from my experience and is not written in stone.

 

I had a K&N on the Rappy before and I always wash and oil my filter before each time out, but I would notice fine powdered dust on the intake boots every time, here's an example of how bad the dust here that if someone is riding in front of you you wont see unless you stay back at least sixty feet and still after a couple of hours I get a bad nasal headache.