Why do you need Turn-Downs on a street car? The performance gain is near negligible. Is your Mustang mainly used as a drag car? If so, than you shouldn't be concerned with warranty at all.
The entire point to full length exhaust is to force the exhaust gases to exit past the passenger compartment. In the Mustang, since the Trunk shares common ventilation to the interior, it's wise to keep the exhaust going all the way back to prevent a slow build-up of harmful carbon monoxide.
In some places it may even be illegal to have exhaust that exits under the car. People can and do die from carbon monoxide poisoning, sometimes from their own and unintentional but tragic end, such as MTV's Buckwild star, Shain Gandee.
While I understand you've done this before, it doesn't mean it was right or safe.
2015 Mustang GT PP 401A Black | Roush Supercharger