I do think mod acceptance varies widely. I've had my Lightnings fixed breaking all sorts of parts with it fully modded and Ford never questioned anything.
The Infiniti dealers down here in Miami just seem to be all-about-stock. They couldn't believe I would swap a sport seat out for a journey. (I'm a big guy). To me, what's the big deal...4 bolts and a couple harnesses later and done!

 

My Infiniti dealership sells and installs Stillen products. I don't think they do a lot of business in it, but it's available.

 

I bet there are dealers that sell/install intakes and other non-Stillen aftermarket parts as well. One way to make sure there are no issues with warranty items is to work with them to buy and install the aftermarket part, complete with a dealer receipt. You can normally work with the parts group and get decent pricing on the part, then with service to get decent pricing on the install.

 

At a ridiculous price too.

I like to save money and just do it my self.