I had a 06 Jeep with 37 inch tractor tires. I went to the dealership to get some work done. They called and said the total would be $5000. I was shocked as the Jeep only had 15k on it and reminded them of that fact. They told me I had 0 warranty. The bigger(HEAVIER) tires causes a strain on the powertrain (True)so that warranty is no longer valid. Jeep did not design the engine/transmission/transfer case to handle that much weight.
I had an buddy who is a hardcore offroader and also a lawyer. He looked into it including the Magnusson act and he said I was out of luck. The magnusson act was primarily written so Manufacturers could not void your warranty if you didnt use "Mopar" oil and a Mopar oil filter. As long as it met the specs you were good for your warranty.
I have heavily modified every vehicle I have owned including a 330+ HP VW GTI. What I have found is it all depends on your dealership and more importantly your service writer. Thats the guy who you want to become BFFs with.