It seemed like the Mercedes-AMG GT R was already plenty extreme, a car conceived for track use but civilized just enough that it can be a riot on the street. But AMG's engineers weren't quite done and upped the ante with the GT R Pro, a thoroughbred track car inspired by the AMG GT3 and GT4 racing cars.

Where else would Mercedes let me test drive it, then, than a race track? Specifically, I spent an afternoon lapping the GT R Pro at the Hockenheimring in southwest Germany.

More of everything

To make the GT R even faster, Affalterbach's engineers amped up every part of the existing car. Gone are the GT R's regular adaptive dampers, for instance, in favor of coilovers with a higher spring rate and manual adjustment for height, rebound, high-speed compression and low-speed compression. AMG provides what it thinks is an ideal setup from the factory, but racers are encouraged to tweak as they see necessary.

More adjustment is available up front, where there's a new carbon-fiber anti-roll bar with two-position adjustment. That also cuts 5 pounds of weight. The rear bar remains a steel, three-position adjustable unit, but on the GT R Pro it's hollow to save weight. More of the car's control arms now use metal "spherical" mounts instead of rubber bushings, to improve precision at the expense of some noise and vibration. Carbon-ceramic brakes are standard and have higher-friction brake pads than the standard GT R, while the wheels are a special lightweight design.

Even more aero

Aerodynamic change are also myriad, with the Pro boasting an additional 220 pounds of downforce at 155 miles per hour than the regular GT R, two-thirds of which is over the front axle. The obvious places where that downforce was gained include a new front splitter, special dive planes at the sides of the front fascia, a new rear spoiler with an extra Gurney flap and vents in the front fenders to help bleed air out of the wheel wells.

Other tweaks include a new carbon-fiber "shear panel" over the transaxle and differential, which improves torsional stiffness by 7%, and a new carbon-fiber roof with a lowered center channel. A total curb weight for the Pro is not yet available, though the regular GT R coupe is listed at 3,594 pounds.

There's new electronic tuning for the dynamic engine and transmission mounts, as well as for the rear-wheel steering and electronic limited-slip differential. And for visual differentiation -- as if all that aero trickery weren't enough -- you can equip the GT R Pro with a stripe package inspired by AMG's racing cars.

One place where no fiddling was done was under the hood, as the Pro retains its progenitor's 4.0-liter twin-turbo, dry-sumped V8 engine and seven-speed dual-clutch transaxle. Output figures remain 577 horsepower and 516 pound-feet of torque.

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Objective straight-line performance figures are unchanged from the standard GT R, with the run to 60 miles per hour accomplished in 3.5 seconds and top speed claimed at 198 mph. But it's on a track that the Pro's upgrades pay off, with the car claiming a Nürburgring lap time of 7:04.6 minutes, versus the regular GT R's already impressive 7:10.9.

The art of racing in the rain

With ambient temperatures hovering at 42 degrees Fahrenheit and rain all day long, it's not exactly ideal conditions for a track test of an extreme car. AMG's technicians have swapped on Pirelli P Zero tires instead of the standard Michelin Pilot Sport Cup 2 rubber, while the company's hired driving instructors admonish us to leave the cars' stability control engaged. Still, the GT R Pro more than holds its own on the slippery circuit.

My first few laps are a battle for traction as I feel out the car on the damp track surface; breaking the rear tires loose at the top of third gear in a straight line does mildly reset one's fear factor. Soon, though, I fall into a quick rhythm, following AMG's instructors in a lead-follow exercise. Acceleration (when there is enough grip) is absurd, the V8 roaring and snarling as I snap off quick upshifts with the paddles. As on the street, the engine's abundance of low-end torque means I can make swift progress even if I short-shift. But keep it pinned to redline and the Pro absolutely flies.

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What is perhaps most noticeable is how well the Pro communicates about its limits going into and out of curves. There's a lot of feel through the steering wheel as the tires begin to push into a little bit of understeer in sharp hairpins, while the quick ratio makes catching up to the sliding tail somewhat easier. Yes, the Pro oversteers easily and readily in the rain, but it does so gently without surprise. The throttle response in Sport+ mode is a little too sharp for my liking, but as I learn to have patience it's easy to balance the car more out of bends.

Grip levels are still very high in the damp, whether when standing on the unerringly firm and easy-to-modulate brake pedal or when holding big speeds through the track's Parabolica curve. Thank all that aero bodywork forcing the car onto the roadway.

By my final session on track, the rain has turned into drizzle, and I've figured out the circuit's layout enough that I actually feel like I can push the GT R Pro a little bit. Unsurprisingly, it is very rapid and very fun. Like the street cars, the AMG's snappy turn-in feels preternatural and its engine is a veritable font of power. So loud is the exhaust at full chat, too, that I can't hear the instructors' notes even with the walkie-talkie volume turned all the way up. And when observing others from the track's empty stands, the crackles on downshifts echo around the circuit like fireworks.

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On a dry track, I might have liked to experiment more with the AMG Dynamics system. It combines settings for the stability and traction controls, rear-wheel steering and limited-slip differential into various modes called Basic, Advanced, Pro and Master -- the latter of which is only achieved in the Race driving mode with ESP fully turned off. But, with such slippery conditions, the Sport+ driving mode was the most aggressive I got.

Still a nice place to spend time

AMG did not, however, gut the interior of the GT R Pro, which is nice, even if it seems unlikely the cars will see much street use. The 12.3-inch digital instrument cluster offers an AMG-specific "Supersports" gauge layout with G-force readouts, oil temperature info, tire pressures and lap times -- as well as flashing red accents when you approach redline. The Comand infotainment system has the AMG Track Pace app which can record telemetry and lap times from your on-track excursions; the data can be exported to your phone or computer for further analysis/bragging.

My test car is equipped with snug carbon-backed bucket seats that save nearly 8 pounds of mass, as well as the optional Track package that fits a roll bar, four-point harnesses and a fire extinguisher. Sadly neither will be offered in the US as the seats and roll bar apparently don't meet America's often fussy safety rules.

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With only a handful of cars available and all dedicated to track use, AMG did not permit me to drive the Pro on public roads. Given how aggressive the standard GT R already proved on normal streets, the new Pro would likely be tiresome to use away from a racetrack. However, because it is still a road-legal machine, with air conditioning, cruise control and so on, it is at least quite useful for anyone who wants to drive, rather than tow, the Pro to their nearest circuit.

A limited, but alluring, toy

With just 750 cars to be built globally and only 150 coming to the US, the 2020 Mercedes-AMG GT R Pro will be a rare track-day machine indeed. Pricing has yet to be announced, but with the 2020 GT R listed at $163,895, it's not hard to imagine the Pro's sticker getting close to $200K. For all the capability on offer here, that's a sum that well-heeled track-day enthusiasts should be happy to pay.

What a rainy day proved more than anything was that John Miessner, head of driving dynamics at Mercedes-AMG, achieved his goal of making the GT R Pro a remarkably approachable car. "Our focus is not only to make a car quick," he said, "It should be possible for even a standard driver to drive it quickly." Given how briskly I managed to get the car around the Hockenheimring, even in sopping-wet conditions, I'd say Miessner's mission was accomplished.

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