I didn't even realize that SiriusXM was only available on the Lariat, that is kind of idiotic. Your Ford salesman lied to you. Adding anything aftermarket would not automatically void the warranty. In order for it to void the warranty Ford would have to PROVE that the aftermarket product (SiriusXM) caused any warranty related issues. There are laws to protect consumers and they can't just void the warranty because you install aftermarket equipment. I can't see how having SiriusXM installed could possible cause any issues. People have aftermarket things added to their new vehicles all the time and the warranty is not voided. Aftermarket remote start is a popular example.