My experience with both the stock and K&N filters is that the stock filter has to be replaced at 24,000 km and was still "decent" at that time, but the K&N plugged up in much less time than that. So, do you want to clean it every 8,000 km and let more dirt into the engine in the meantime, or replace it every 24,000 km and not concern yourself with it in the meantime?
Take a look at a K&N filter and a stock filter side by side, and compare the number of pleats. The stock filter has much more surface area, that's why it doesn't plug up as fast.
With both filters clean, I noticed no difference in how the bike ran.
YMMV of course. The ram air on my bike in this area scoops up all sorts of bugs, mosquitoes, leaves, etc., and the roads are generally dusty for a lot of the year due to winter sanding and salting.