Just ask your dealer. That would be my first step. Agreed they would have to make a good case your mod caused any future issue but why not see beforehand how much fighting you might be in for IF a problem came up. And since you can get a 2015 R/T with a 392 Hemi and a shaker you have an awesome fall back plan....
I'm sure it won't be a problem, they would have to crazy to try to void it over a cat-back mod. And nahhh, I like the 6.4l and all, and I'm sure with the new ZF-8 8-speed transmission it is going to be a ROCKET vs that old NAG-1 5-speed. Peak power any time you mash it lol +200ms shifts I think 150ms if you put it in sport mode)
But you know I'd just wait 3-4 years and trade in the Shelby towards the 707HP Hellcat Challenger that someone bought and just garaged, once you get used to the super-charger whine, man I can't imagine not having a super charged car. It's addictive.