OP...do what you like. Just be straight-up with folks as to what it is and you'll be fine. Noone on here (myself included) should sway you one way or the other with our opinions. That said here's mine. You've got enough Nismo **** on your car that badging it is fine in my book.
IMO it's not the same as GT500 or an M because the Nismo upgrade, if you can even call it that (I've got one) is not comparable to those upgraded packages. I love my Niz, but I don't think that mine is better than a non-Nismo Z (better looking yes, but taste is subjective), but performance wise, not really. They could've/should've done better. What my Nismo is that a non-Nismo with exhaust mods isn't, is legal in CA...and since my goal was not to mod, the Niz is what I went with...and I couldn't give a rat's *** what anyone has to say about that. Plus I don't have to take **** off every time I have a dealer issue. But to each his own.
Putting a Nismo emblem on your car given all your Nismo mods IMO is definitely more relevant and appropriate than putting a Fairlady Z badge on any USDM Z car, which many of my friends have and it looks fine and I'm not hating...you do you, you Fairlady Z badging people! But if people want to get picky and give you **** for badging your car with a Nismo emblem, if it wasn't built as a right-hand drive car for the JDM, then illegally imported into the US, it is not a Fairlady and shouldn't have the Fairlady emblem. If we're going to say that you're rice because its not "a real Nismo from the factory" then anyone with a Fairlady emblem in the US that doesn't meet the conditions above is automatically even more rice than you.
I'd be interested in hearing what Waizz, our resident Nismo freak, thinks about all this badging business. This whole discussion and argument just cost me about 20 IQ points! Damn my head hurts. Do what you like man! Good luck.