I disagree and then agree.
This is MY feelings on the topic, by no means the cold hard fact: I feel, I the dealer is presenting the warranty plan (the buyer did not seek it on their own), then I feel they have an obligation to their customer to ensure that the customer is either getting full benefit of the plan OR is made aware of issues (such as items not covered with the car in the condition the customer is buying it in).
Sadly, as stated, not everyone is going to know that the car has aftermarket parts. And especially if this is a dealership that plays the "certified used car" bit (giving customers the warm and fuzzies the car was completely inspected), again, my personal feelings are the dealership has skin in this game and I would do my best to hold their feet to the fire to either make things right or for mis-representing options to me.
Again, my feelings towards the matter based on the information I am reading in this thread.

Are you saying the "aftermarket exhaust" is a GMPP exhaust? If so, sure, it's NOT original equipment, BUT, it is an exhaust GM markets towards these cars!! Considering it's a GM branded exhaust, I really think it's silly the shop even mentioned damage to the drive train.

I hope things pan out to a resolution you are happy with. But if it is truly not a covered item, you may be in a position to compromise with the dealer. But as I mentioned, if that exhaust was on the car when they sold you the car and offered this warranty, then they have some skin in this game.

 

Yes you can buy G M warrenty for your car, its this finding the right dealer that will deal with you on this. I have G M protection plan-Major Guard. $100 ded. and cover till 2-28-16 paid $1800 for the plan. Its a low mileage plan cause i am only putting on about 7500 miles a year on the car.

 

More proof that dealers in general, do not know much about cars. They tell you anything you want to hear just to get you to sign that contract.

And the mechanic - Umm yeah, the backpressure causes wear on the valvetrain...what?

The Magnaflow aftermarket exhaust system for the 2007 Monte Carlo SS is 100% Stainless Steel, and will not "go bad."

Good job on being persistent with the dealer and making them own the warranty. Looks like everything turned out just fine...