There are many high powered Cabriolets on the market but the fastest and most powerful goes to Mercedes-Benz that has been customized by Brabus. The vehicle made it's debut in France at Les Mans.
The Brabus 850 6.0 Biturbo Cabrio has a 5.5-liter twin-turbo V8 engine that gives out 577 horsepower. It can reach a top speed of 217 miles per hour and can go 62 miles per hour in 3.5 seconds in a stand still state. The power comes from new forged pistons, exhaust system and turbochargers. Some interesting information from Brabus is they say the vehicle can go even faster but the tires just aren't able to handle it.
It should be noted that the Bugatti Veyron Grand Sport Vitesse and the Hennessey Venom GT Spyder are faster than the Brabus 850 6.0 Biturbo Cabrio but those two vehicles don't have cabriolet-style folding roofs which puts them out of the conversation.