You gotta oil them, same as foam, but yeah they are gonna let in more debris because the media is super thin. Foam does a much better job. There is a reason every single dirtbike being made today uses foam.
If you buy a good name brand dual layer foam, like a uni, it will keep things pretty clean.
Are you planning on upgrading carb and pipe too? I dunno much about the qt but Yamaha usually has a pretty free flowing airbox. The pw seems to be restricted with the exhaust.