I read the test that you linked to, that was quite interesting. This sentence really caught my eye-
"Compared to the AC [OEM paper filter?], the K&N “plugged up” nearly 3 times faster, passed 18 times more dirt and captured 37% less dirt."
However, the test results also showed that the AC filter was the most restrictive, but the discussion argued that it could pass all the air through it that the GM diesel in the test could use, at least when it was clean.
As I said above, because I was concerned that it would not filter effectively enough, I did not use the Mountune branded K&N type air filter that came with my Mountune MP215 kit. I chose instead to keep using an OEM paper filter with the new air box that comes with it, and I'm just going to change it every year, which is only about 4,000 miles for me, so it will always be clean.
A lot of comments on here from knowledgeable members led me to believe that the OEM paper filter would work just fine with the new airbox. The fact that Ford used the OEM paper filter in the European Fiesta ST200, which has an identical airbox to the Mountune one except that it's plastic instead of metal, also was convincing to me.