LOOK hard, and then look harder again, because the E89 Z4 is not quite what it appears. Sure, the soft top has been turfed out for a (heavier) folding hardtop roof, and the interior now reeks of quality and luxury, so thoughts of a Mercedes-Benz SLK challenger instantly spring to mind. But the flagship sDrive35i has a formidable ace up its sleeve in the delicious form of BMW’s fabulous twin-turbo 3.0-litre powerplant, and this fact alone raises hopes for a two-seater roadster that can finally dislodge the long-reigning Porsche Boxster as the world’s greatest convertible. Has BMW succeeded?