I don't think there are any real differences in the muffler section there are a few members here to gutted the stock muffler. The real difference in the Nismo exhaust in how the tubing runs. Stock Z's use the Y pipe design and Nismo's use an H pipe. the differences are so negligible that its hardly worth the investment unless you like to say "I have a Nismo exhaust."
Personally, with so many aftermarket exhausts up for sale new and in the classified, why would you get a Nismo exhaust? Not worth it IMO.
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2009 Carbon Fiber/Red 370z aka Trish: Sports Package, Meisterschaft full catback, Fast Intentions Longtube Headers, Injen Long Tube Intakes, UpRev Tune: Sold 
2008 Lotus Exige S Club Racer aka Bella, always leaves me smiling
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