The proof is in the spy photos above. Around the side, it has the GT R's fender vent with a single bar across it and strakes that extend into the doors. The wheels and brakes are identical to the GT R, too. And guess what? The rear is also straight off the GT R with the big rear diffuser, center-exit exhaust, and unmissable manually adjusted rear wing.
Up front, this roadster has the wider, more aggressive GT R front bumper. However, after zooming in on the lower grille and fiddling with shadows in Photoshop, it's hard to tell with the active aero shutters have been replaced with fixed slats or if they're just open.
We expect that this car will simply be a GT R with a soft top, meaning it will have the same twin-turbo 4.0-liter V8 making 577 horsepower and 516 pound-feet of torque. It will also have the GT R's 7-speed dual-clutch transmission, rear-wheel steering, electronically controlled limited-slip differential, adjustable traction control, adjustable coilover suspension and carbon fiber parts. We're not sure if the same aerodynamic trickery used through the coupe will work the same when the roof has been removed, so perhaps some of those elements may be different (such as those slats).
Being a convertible, though, it will probably be heavier because of chassis reinforcements and soft top equipment. As a result, it will likely be a tad slower, but you'll also be able to hear its wicked exhaust better. The car will also probably appear soon, possibly by the end of the year, or early next year. This is based on how little camouflage is on the car, as well as the fact that much of the powertrain and chassis has already been developed for the coupe.