We have 100's of Nissan dealerships across the country that trust and install STILLEN products, a lot of times onto vehicle before they are sold.
If you're worried about it, have the dealer do the installation.
That being said, you're backed by the Magnusson-Moss Warranty Act, which specifies that an auto manufacturer cannot force you to use exclusively their own parts and void your warranty. We did a write-up of some additional information, this was also validated by the FTC in 2011:
Federal Trade Commission Validates Right to Install Aftermarket Parts Without Voiding Warranty : STILLEN
If the aftermarket part in question fails, obviously the warranty will not apply to that part, but not the whole warranty. You're not alone in this, there are thousands of Z's that are modified, and routinely go in for services at the dealership.