If you've been waiting for a luxury convertible with enough power to rip the hair from your scalp, wait no more: Mercedes-AMG has you covered. Mustering a frankly indecent 737 lb-ft. of torque, Mercedes-AMG S 65 Cabriolet mixes together four seats, a fabric hood, and a huge 6.0-liter V12 engine. That means a total of 621 horsepower courtesy of twin …