The K&N belongs in a race car, where the goal is optimizing full throttle operation, and pressure drop is important. The C-Max isn't a race car, rarely runs unthrottled, and is designed to minimize fuel consumption. Doesn't sound like a good fit, does it?
Since you've had that K&N in there a while, the proof should be in your oil. What's your oil's Si level after 10K miles of low-efficiency air filtration? Mine's been dropping and klatoo's Si levels are similar to mine (when normalized for oil mileage) so there's the beginnings of a basis for comparison.
I forgot about this thread, but it's worth mentioning I have at least 60k mi. ICE miles on my K&N Air Filter and I got my best Oil Analyses Report yet. Not sure that it improved MPG's, but It seemed to me it improve acceleration. I guess I could put my New Ford filter in( what a job ) And see if I notice a difference.