IMO, the K&N filter are not good for turbo powered vehicles, as there is much more suction. When the oil eventually disappears, dirt gets through and leads to damage/wear...the term is called "dusting". I'll stick to dry filters myself. I'm not sure about tbe 6.4L, but the stock filter in my truck (6.0L) has one of the best ratings for filtration, and is good to about 500hp...I'm around 450 now and don't plan on much more.
The Hauler - '05 F-350 4x4 CC Dually 6.0 Powerstroke
The Project - '90 Ford Bronco 4x4
The wife's - '11 Ford Escape
For every minute you are away from work...you can add two minutes to your life!