No modification to the exhaust after the Rear O2 sensors will void warranty unless Ford engineers can prove that your modification has directly caused the damage.
For example, if you crush the tail pipe and the engine is damaged, then the crushed tailpipe is likely the culprit. Ford won't cover that.
2005 GT Premium - Mineral Grey 5-speed MT
Vortech Supercharged - Steeda Billet LCAs, Steeda UCA, UPR adj. Panhard bar, FRPP K-member brace, GT500 Strut Tower Brace, Walbro 255lph Dual pump, FRPP 39# injectors, AIS Water/Meth Injection, Ported Throttle Body. 440rwhp / 406rwtq