I have logged every mile and every gallon of gasoline my NCX has covered (closing on 50k miles). The K&N measurably negatively impacted fuel economy over a long period of time. Going back to stock restored mpg. It doesn't matter what theory anyone advances, the observed fact is that air filter differences _can_ and sometimes _do_ impact MPG. Accordingly, they _can_ impact performance.
If I still have that K&N, you're welcome to it for $25 shipped to the Lower 48 States.