I test drove a 15 too. Also have a 12 brembo. Your experience same as mine. The 15 gt felt kinda soft, in a luxury car way. Keeping my car for now. Also not too excited about the way the 15 looks.
2012 Mustang GT Brembo, 3.73, all weather mats, rear parking sensor, Gibson Stainless Axlebacks. 300a. Rousch takeoff leather interior, Boss 302 steering wheel. Mustang Club of MD.