I used K&N filters as well as similar type filters on different motorcycles and was quite happy with them. Compared to paper type filters, I don't think K&N type filters will be any better though.
Specially the "racing" K&N type filters with a claimed "30 %" (or whatever percentage) "better" air flow, might be a bit risky.
I think any filter who allows "easier" air flow, will also allow more dust into an engine. (I.e. - a filter which allows a 30% better air flow into the engine, will probably allow about 30% more dust into the engine as well).
A thin layer of grease get smeared at the inside of all my vehicle's air filter box housings - including at the inside of my scooter's air box. In my scooter's case (Agility 125), all dust get trapped in the grease - and dust or dirt doesn't even reach the modified filter I made as replacement for the original filter.
I wouldn't be worried about a K&N filter - specially if the inside of the filter box, as well as the inner sides of the inlet pipe to the filter box get covered with a thin layer of grease as well.
