Should I upgrade to a nismo?

FL 4Motion

In a word, no. That is a financially stupid move. That said, if $$ isn't an issue, then you really need to test drive a Nismo model and see if it floats your boat better than your current Z. (btw, WHY the hello didn't you do that in the first place and save yourself this issue???)

The 370 nismo isn't seam welded like the 350 nismo b/c it doesn't need to be, the 370 is already vastly stiffer chassis over the 350.

the 370 nismo bodykit is functional windtunnel tested aero, hard to duplicate that with aftermarket without spending $$$. If tracking the car isn't an issue for you though, then I assume you just like the way it looks.

The wheels are forged lightweight rays that are wider and lighter than the standard sport package rays. To get aftermarket volks of the same quality, you'll be spending $$

It's only 18hp thru a tune and and exhaust, so powerwise, not much difference.

The nismo suspension is stiffer and more "track ready" than the sport package but it also rides much rougher, something to think about.

All in all, I wouldn't do it simply for the financial loss, however, I get wanting something so bad sometimes you don't care about the $$. In that case, get what you want but use this as a lesson to think thru your purchases thoughorly so you don't wind up buying twice.