Here's what you need to understand about warranty stuff. Unfortunately the provider of the warranty is in charge of the situation... regardless of what you think is legal.
You will have no case if you ever took them to court. First off, your court costs would be more expensive than paying for your repairs. Secondly, they have tons of lawyers and "Experts" to examine your engine and tell the court what damage your bolt-ons cost.
Here's a true story to explain what can happen.
Man puts on Cold Air Intake. 4 months later, engine blows. Takes car to dealership (not Nissan but applies to all). Dealer says warranty void due to modification. Man decides only course of action is to take car company to court. Man looses his court case... Company experts and examiners say that the added torque was more than transmission was designed for. As a result the faulty tranny caused undue stress on the engine and caused it to overheat and blow up. Man lost court case and had to pay $3000 in court fees plus $5000 to replace engine.
Now, I am not telling you that you shouldn't mod your car. I think you should, and the more the merrier. What I am telling you is to be careful, and think twice about your interaction with Nissan. If they can avoid payment in any way, they will. A lot will depend on the service writer. The service writer can push things through on warranty for you if noone else sees what happened. You just have to get in good with them.