Johnny you are correct, that warranty (or lack of one) is on the NISMO Parts themselves.
The point I was trying to make is that you can actually add NISMO performance parts to the 350Z as you build it on the NissanUSA web site, so it would seem to be a double standard if they tried to void your warranty for adding NISMO performance parts to your Frontier. I know they can get pretty creative with void portions of your warranty (I used to own Fords too), but armed with information from their own web site and the Magnuson-Moss Act, a good lawyer would be able to make a pretty damn good case against them, especially if the parts were purchesed through Nissan (yournissanparts.com is Nissan of Turnersville, MJ) and properly installed, reqardless of who installed them!
Bottom line, if they failed to properly test Genuine NISMO parts that they made for Nissan vehicles, then shame on them! The arguement could be made that if they knew they would cause damage, (especially the S-Tune Parts) then why did they sell them?