Pretty much anything requiring tune will void warranty in my experience. I have a jlt resonator delete tube, borla catback exhaust, bmr uca,lca mgw gen2 shifter and no issues with warranty claims.
I purchased an extended warranty because I didn't plan on doing anything tune related or really any modifications which would require tuning. If you're putting a lot of miles on the car and plan to keep it stock then I don't see any reason why not to purchase an extended warranty.
You can get a prorated amount of money back from Fords ESP plan if you decide to sell or modify the car, the plan is also transferable to a new owner.
This is where I bought mine. Flood Ford