dogdamn, i accidentally deleted my post..

it's really the muffler dat's restrictive on the catback. unless u go turbo the stock pipes are fine imo.

Agree... the mid pipes are fine with stock motor.

while the muffler is more restrictive relatively, it is probably also ok with stock motor. most of the hp gain with aftermarket muffler is way up on the rpm band and might even lose a bit on the low end.

If you are looking for sound with a slight performance increase go with a NISMO (less expensive) or aftermarket mid pipe and keep the stock muffler (it's louder than the NISMO muffler). This will make the car sound significantly better without the cost of a full cat back.

'14 Nismo is much too quiet
 

I was very disappointed in the sound of my '14 Nismo. The exhaust note was a short step above a sewing machine - I couldn't really hear it at all while driving. I never heard it from the outside because I never quite mastered the art of driving while outside of my car. Besides I don't really care what everyone else hears - only what I hear inside the cabin.

I absolutely love the HKS. A Nismo should sound like this from the factory. Absolutely no drone and sounds fabulous when you accelerate, especially with the windows down.

Take a listen:

HKS Hi-Power Exhaust