Cold air intake modifications are generally available for American cars because of the power obsession ;-). The Japanese market loves customization, so we might see them do more for performance.
The K&N filters made a lower and less smooth noise in my installs. On one vehicle I couldn't notice a MPG gain, and I'd have no dyno metric for performance, but it "felt" better (a very reliable and useful benchmark for someone who spent ~80$). The other install included a carburetor change that lowered MPG significantly, but switching the air filters back and forth made an extremely obvious difference for intake noise and performance.
Dyno tests generally show a few HP gain, nothing noticeable. MPG can go up a couple numbers, which is noticable. On a performance cars like Infiniti cars, it's unlikely they are restrictive (I'd have to check). I think air intake is generally underrated, but gains from modifications are generally overrated.