Not sure what you are reading but I didn't say anything of the sort. I said the stock filter isn't that restrictive so there is little to be gained by using a different filter on an unmodified engine. Never said K&N filters restrict airflow, what they don't do is filter the dirt particles as well as the OEM filter - because they are less restrictive.
The drop-in filters do add a bit of noise, certainly not as much as a cone filter but they do add a little bit compared to the OEM filter.