That's nuts.
Is there really no baseline? No, there IS no baseline
Seems like there has to be a consensus at some point, a local, regional, national whatever, they have to have drawn a line somewhere? Nissan follows the Magnuson-Moss. Each dealer chooses how aggressively to fight for you. If they tell Nissan that you abused it, you'll get nothing. If they go to bat for you and do the work under warranty you're all set. If Nissan decides to not reimburse them, they bear the cost. Larger dealers carry more weight with the OEM.
I also know you can buy a Stillen SC and if it's dealer installed it will not void the warranty but if it's installed in your garage you're on your own.
not entirely true. First they can only try to refuse parts that would be directly damaged by the installed part. If the dealer installs it AND gives you a warranty with it then yes. But they can install it and treat it like they were a 'regular' garage and therefore leave you to fight the MMAct
It really comes down to the franchise/dealer owner policy. Case in point. My backup car, a 04 Hyundai Elantra, was having some "issues" where it would refuse to start on some (cold, damp) mornings but if I tossed a space heater in the driver footwell for 10 min it would start right up. Went to the nearest dealer (Ira Hyundai in Saco ME) and they found & fixed it no problems. While they had it, they noticed my electric mirrors were iffy. There was a recall or something identifying the door harness plug as not being waterproof enough up to early '04, but mine was not part of that action. They went ahead & fixed it for me their dime. Their fix caused other issues because the new connector/harness they tried to wire in didn't have the same color codes as the old harness in my car. All told it took them 3 visits and 6 weeks (about 120 hrs billed) to get it done right by manually tracing everything out. They gave me a rental car unlimited miles each time, and didn't charge me a dime.
Great experience
Few years later, relocated for a job. Had an intake manifold gasket fail (under warranty). Local dealer (Autofair Hyundai in Manchester) tried to refuse warranty and bill me $800~ to replace the gasket because I was within 1k mi of the end of the warranty.
Bad experience.
It all comes down to the dealer's attitude.