Unfortunately, air filter threads are nearly as bad as oil threads, and here I am contributing to what will likely end in controversial thread. I apologize in advance.
Do not use racing filters as there is no need for more air flow on the NC, and the racing filter will likely trap less dirt. The NC700X air filter is quite large relative to the engine size. Adequate air flow is not a problem.
I buy genuine OEM Honda filters from the online part seller with the lowest price. Since I ride in fairly dust free areas, I have no problem going at least 24,000 miles between air filter changes.
If I had a K&N air filter for the NC700X, I would give it to you for free.
I stopped using K&N filters after my experience with my Goldwing. The properly oiled filter pictured below had 35,000 miles on it. It was hardly dirty at all, presumably because it allowed most of the dirt to pass through the engine, instead of trapping it. The pores of the K&N filter are so large that I held my iPhone camera up to the dirty filter, and took a pretty reasonable photo of my garage window THROUGH the used air filter. This suggested to me that the K&N filter is so open that it would fail to trap dirt and protect the engine. I tossed the K&N in the trash and I will never use another one. It's my belief, based on my observations, that K&N oiled air filters do not adequately filter dirt to keep it out of the engine.
Photo taken looking through the dirty filter.