Mercedes-AMG is expanding its GT family with two new Roadster models: the GT Roadster and the GT C Roadster with a performance level between the GT S and the range-topping GT R. With these 2 new variants, the GT family now hasto six members. Outputting 550 horsepower, this 2018 GT C Roadster is stuffed with a 4.0-liter biturbo V8 driving the rear wheels through a 7-speed dual-clutch transmission. Taking cues …