I doubt very much that any dealer would void your warranty if you had an exhaust system installed by Midas Muffler on your truck. Functionally there is no difference between a Midas exhaust and one from Gibson, Magnaflow, OE, etc.
That said, I did put a Gibson cat-back on my Sport Trac when it was practically new. Now I leased it when gas was well over $4.00 a gallon so it was a measure taken to help mpg, which it did. I intended to buy it when the lease was up, which I did. The service department presented me with a $600+ bill to "Replace non-stock exhaust system" from the post-lease inspection which the sales department promptly threw in the trash since I was buying it anyway.