Mercedes-Benz has traditionally treated its flagship coupes with buttoned-down restraint, incorporating precision engineering into an understated package that flies below the radar of most driving enthusiasts. For 2015, the cars in the Mercedes-Benz S-Class Coupe family depart from that paradigm, with all-new designs that feature more aggressive and even sporty characteristics than the German manufacturer typically employs. Included in the new line are the S550 ($119,900) and S63 AMG ($160,900), which both off…