The most obvious additions are up front. There are twin canards on each side of the front bumper, with the upper ones cutting right across the outboard grilles. The front splitter is much larger now, too, and it has a pair of adjustable supports mounted toward the middle.
Moving back along the fenders, we find that one prototype has extra winglets mounted above the fender vents. While they're certain to be effective, they do look a bit strange, so we're not totally sure they'll make it to production. One other prototype caught at the same time lacked the winglets, but had large vents along the top over the tires. That seems much more likely for production.
At the rear of the car, the wing appears to be the same adjustable unit as found on the regular GT R. The rear diffuser looks about the same, too, but the exhaust is different. On the current GT R, the exhaust is routed through the center, but here, the exhaust is split and sent to either edge of the bumper.
Since this more serious Mercedes-AMG GT R appears to be restricted mostly to aerodynamic changes, and doesn't radically change the design of the car, we expect to see it soon, probably for the 2019 model year. In addition to the exterior updates, we're expecting it to have more track-focused suspension. It may have more power, too. It's certainly not impossible, since the twin-turbo 4.0-liter V8 makes up to 630 horses in the four-door. We'll have to wait until the car's reveal to know for sure, though.