Any dealership can screw your day up with modifications. Weak case or not, what do you do when they refuse to help you? Sure you could be in the right, you could try and sue them, do you have that time and money?
Modifications: either do your own maintenance, take the
mods off before going to dealership, or be ready to pay to play. Or you could do what I did, and just ask your dealership what they are ok with. Intakes, catback, no problem will never question it. Some dealerships wont be that cool.
Theres plenty of threads about this all over the internet, its probably one of the first forum posts when the internet was created. This question is not specific to the Stinger/
G70 or any other car. Plenty of stories going one way or the other, its up to each person to decide their own risk/reward threshold