If you look at the K&N filter for 6th (and 5th) gen VFR you'll notice that the actual filter size is much smaller than the factory filter. The K&N has about half the surface area of the factory filter. K&N knows that because the bike doesn't have an airflow meter it has no way of accounting for any extra air flow. K&N makes their filter flow the same as stock so as not to disturb (further) the air/fuel ratio. Since K&N's filter medium flows more than the factory paper filter, they simply made the filter area smaller to even things out. Which means, by design, the K&N filter buys you no performance gain, zip, zero, nota. However, it's the last filter you have to buy because you can wash and re-oil it.
K&N Filter
I also don't like K&N's design for the VFR. By making the element smaller, it's now centered over the rear throttle bores while the tops of the front bores are covered with plastic. On the factory filter, the element protrudes up into the air box cover. The K&N element protrudes down into the airbox. These two things would make me believe the front cylinders would have a harder time getting air and would be rather rich compared to the rears.
Beside, Frankenfilter was almost half the price and you get that factory fit.