Driving it and over forty years of cars on the street and race tracks.
Anyone coming from a Porsche is going to want to switch to Konis, Bilsteins, whatever......the stock units don't have enough damping. Of course that's if they want their Mustang to handle close to as well as their Porsche. If they're buying a Mustang to get a softer, smoother ride, then I would imagine the stock units would be fine.
2012 GT Premium, Brembo PP, MT82, 3.73s, Blk/Blk. Bama 93R, Koni Yellow Rear Shocks, Hawk Ceramic pads, Luxurious & Plush Floor Mats