By itself, comparing the K&N to a fresh HD OEM air cleaner element of comparable size may see 10-15% more air flow...perhaps more over the same amount of time as more dirt gets trapped.
The real key is that when you actually change the aircleaner element AND housing, the larger exposed area to open air, means less restrictive element over area. Heavy breather works by virtue of totally open perimeter, filtering area is a bit smaller regarding filter element area, but certainly more than stock.