Arizona State Route 89A lies roughly 100 miles north of Phoenix, and as this twisting bit of roadway stretches out ahead of me, it promises exactly the sort of breathtaking mountain and valleys views that I need after a long Michigan winter. Pausing at one of the pull-offs for photos wouldn't be a bad idea, but today, the Copper State's natural scenery will play second fiddle to the 2018 Mercedes-AMG GT C Roadster that I've come here to cane around.Open-top versions of the Mercedes-AMG GT sport car family are the latest members to join the fold, following the company's track-focused GT R. If there was any worry that the roadster was going to give up anything in the performance department compared to its coupe brethren, those thoughts are quickly squashed on Route 89A. With its soft top down, the GT C Roadster confidently hunkers down through corners, handles quick side-to-side transitions without noticeable body roll and doesn't exhibit any signs of understeer -- even through the really tight stuff. Benz's latest convertible does everything I ask it to, and it's undeniably engaging behind the wheel thanks in part to steering that's responsive, weighty and communicative. This roadster's agility can be traced back to its chassis, which is treated to thicker side-skirt walls, additional dashboard- and windshield-frame reinforcements, plus a rear strut tower brace. A crossmember behind the seats to support the roll hoops is also added to help stiffen things and make up for the missing fixed roof. To raise handling capabilities even further, the GT C poaches hardware from the GT R's parts bin, including its active rear-wheel steering system, electronic limit-slip differential and adaptive damping suspension. Compared to the base GT, there's also rear bodywork that's 2.2 inches wider to accommodate a longer track and larger 20-inch rear wheels.Things have changed under the hood compared to the standard car, too, with its 4.0-liter twin-turbocharged V8 receiving a different set of pistons, fuel pump, unique exhaust port machining and turbos pushing 18 pounds of boost instead of 16. This results in 550 horsepower and 502 pound-feet of torque, with the latter available from 1,900 to 5,750 rpm, making for a nice, wide torque band. With all that power, Mercedes-AMG says my test car hits 60 mph in a very believable 3.7 seconds, and it'll charge on to a top speed of 196 mph. The GT C's V8 enjoys zero turbo lag, with great slugs of power available everywhere throughout the rev range. Hustling out of corners and surpassing posted speed limits on the straights is accomplished alarmingly quickly, and through the standard AMG performance exhaust, this powerplant sounds muscle-car good at wide-open throttle. Partnering with the blown V8 is a seven-speed dual-clutch gearbox with roadster-specific calibration and ratios. The Getrag-built transmission is fantastic in manual mode, promising near-instant upshifts and impeccable rev-matching for downshifts. Bleeding off speed on my GT C Roadster tester are optional carbon ceramic discs, and the touchy brake pedal that calls upon them requires a brief period of acclimation. Once coming to terms with the aggressive bite, these brakes become my best friends, letting me confidently dive deep into corners at will.Mountain driving sees me switching between Sport Plus and Race settings on the AMG Drive Select system for optimal steering, damping, engine mapping and transmission shifting for spirited driving, but when going through city traffic and straight expressways, I call up Comfort mode. Steering loosens slightly, transmission shifts aren't quite as instant and more give is provided by the suspension. The GT C Roadster still feels stiff, but there's enough damping for relaxed Sunday drives and weekend road trips, at least out here on the region's smooth, sun-baked roads.The majority of my day is spent with the top down, and for the record, Arizona can still be rather brisk in the morning. Thankfully, my car has heated seats and standard Airscarf neck heating vent to help take the edge off. Overall, the Nappa leather-lined interior of the GT C Roadster is an enjoyable place to be with supportive seats, a comfortable driving position and minimal wind buffeting in the open cabin -- even at very high speeds. To handle infotainment functions, Benz's COMAND system remains, with its central dial controlling navigation and the Burmester audio. On both the GT and GT C Roadster, a more powerful Burmester high-end surround sound system will be available as an option. If you don't fancy COMAND, you'll have to learn to tolerate it for now, because the AMG GT Roadsters are not yet capable of running Apple CarPlay or Android Auto.In addition to standard tech like a rearview camera, parking sensors and Collision Prevention Assist Plus to help avoid front end accidents, an AMG Track Pace feature is included on GT Roadsters. With an Apple iPhone app connected to the car's COMAND infotainment system via Wi-Fi, owners can record track lap video and car data points including speed, gear, steering angle, time and position to share with friends and other AMG drivers on Facebook and YouTube. At the end of the day, I finally put the GT C's top up while sitting a stoplight. At the push of button, the three-layer soft top is up in 11 seconds. With the cabin enclosed, it's credibly quiet, with only a little tire noise seeping in from the wide, grippy Continental SportContact 6 rubber. Both this GT C Roadster and the standard 469-horsepower GT Roadster go on sale this fall. Pricing isn't available yet, but expect the GT Roadster to be a bit more expensive than the $111,000 AMG GT coupe, and the GT C to carry a commensurate price tag above the $131,200 GT S. After spending 300 miles and the better part of a day in the GT C Roadster, it seems to me that Mercedes-AMG has put itself in prime position to give topless Porsche 911 variants a run for their money. Right now, I don't think there's a 911 Cabriolet on sale that I'd rather have had at my disposal when heading back out onto State Route 89A.The GT C really is that good.