Actually, the '13's are rated at 420 HP. Did you dyno the car in 5th gear? We need your SAE numbers.
'12 Ingot Silver Mustang GT Premium, 401A, MT-82, BMR Springs(front), Eibach Sportline Springs(rear), Koni STR.T's, FRPP GT500 Strut Mounts, J&M Adjustable Pamhard Bar, FRPP 3.73's, CS Front Valence, 19" x 9.5" Hypersilver OEM '10 GT500 Wheels. LMR Shorty Antenna