The all-new Mercedes-AMG CLS53 was launched at the Detroit Auto Show at the beginning of the year and, a couple of months later, made its European premiere in Geneva.

Since then, quite a few people sat behind the wheel and gave their feedback on what the hybridized performance executive four-door coupe feels like.

Naturally, one would assume that development has ended. But then why did the Affalterbach-based outfit took two prototypes of the car to the Nurburgring recently?

The white one looks showroom-ready, but take a closer look at its grey sibling and you will notice a roll cage in the back. We don’t know what this particular car was used for, but we guess that AMG wouldn’t wait until the 11th hour to set up the CLS53.

Order books for the range-topping CLS will open in just a few weeks across Europe, and it will arrive in the United States later this year. In both markets it will use the same hybridized powertrain consisting of a turbocharged 3.0-liter inline-six petrol engine and an EQ Boost starter-alternator providing the juice for the 40V electrical system.

The total combined output is 435PS (429hp / 320kW) and 520Nm (384lb-ft) of torque, and is channeled to both axles through a nine-speed automatic gearbox. With the new generation, Mercedes won’t offer a CLS63, so customers will have to make do with the 53’s 0-100km/h (0-62mph) sprint time of ‘just’ 4.5 seconds and a top speed of 270km/h (167mph).