Power. Every one wants it and you can never have enough of it, even in a GT. Shorty headers allow exhuast to flow out of the engine with less resistance. Less resistance means less pressure and and increased horsepower and torque. Getting Ford’s headers also means that they match up with the original hardware.
As an added bonus, headers will give the Mustang a definite rumble. Power and sound? Sounds like a win-win to us.