The fastest sport luxury sedan car in the world that is streetable has to be the Mercedes-Benz S600 Brabus. The power plant cranks out prodigious amounts of power so that it has a top end of more than 210. The power plant cranks out 750 horsepower. The number two has to be the Porsche 911 Panamerica Coupe, whose twin-turbo 500 horsepower engine gives it a top end of about 190 mph. The third fastest of this luxurious trio is the Maserati Quadrupole Sport GS, whose turbocharged 440 horsepower engine cranks out prodigious amounts of speed, with an almost 180 mph top end. Remember that like the M-B, each of these vehicles are streetable or street legal, so it is possible you might see this pass you on the interstate.
For more info, see Top 5 Luxury Sedans