I think most people, including myself, do not modify the exhaust for sound only. Most people will change or modify the exhaust for better flow and to eventually make more power. Like with everything in this world where you can devote more time to a single piece of work you most of the time get a much better product. As much research and time as Nissan spends on the car they have to do it for everything, whereas an aftermarket company can spend more time perfecting a single aspect of your exhaust.
I chose to go with my after market system because I needed 3.5" to support further mods, however I did not need 4" even though that would have been louder. Most of the time cut-outs also produce a very unrefined sound and to some of us the sound and how it develops is very important. Some of the after market vendors literally invest thousands of dollars into proper sound design and wave shaping.
No they don't and thousands of dollars in sound development is peanuts. I do agree that a cut out is unrefined sound but you don't beat a cut out for better flow. No better pressure difference than dumping to atmosphere.
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