Congratulations! - Dr. Chubbs can see you've clearly developed acute Zeditis already.
As for the idea of getting a new exhaust, does the new Nismo have fake engine noise these days, like the standard car? You'll know if it's got the fake noise by looking at the roof headlining, above the driver's head.
If you see one of these little microphones...
...you have fake engine noise. If you see one, there will be two others - one in the lining above the passenger's head and one towards the rear. If they're nowhere to be seen you don't have the fake engine noise or, for that matter, active sound enhancement either (road noise killer).
If you have the fake engine noise and you want to turn it off (because you don't like it, or because you want a loud exhaust instead), you can't - the dealer has to do it!
I'd be interested in clarification of whether or not the Nismo has it. It's a much maligned solution to the lack of V6 grunty sound in the 370, but I must say, having lived with it for a few months, I don't mind it in most circumstances. I'm pretty sure the American Nismos have it.