What is it? The new Maserati Gransport Spyder is, according to the sales bumph, “the most powerful convertible in its market segment.” What? Unless Italian horses pull harder than German ones, that means it must have more oomph than a BMW 650i, a Mercedes CLK63 AMG, even a Porsche 911 Carrera S. Well, would you have believed it?They’re wrong, of course. Despite having been given the go-faster Gransport treatment, Maserati’s convertible with its 4.2-litre flat-crank V8 knocks out 396bhp at 7000rpm, up from 390; the CLK63, with its 6.3-litre V8, pumps out 474bhp, and also costs just shy of £70k.But never mind that - as was the case when the Coupe got the Gransport treatment, the Spyder has been improved in other ways, too. Every one of them gets Maserati’s Cambiocorsa paddleshift gearbox, which has been tweaked for faster shifts, as well as an extra helping of leather and carbonfibre lavished on the interior, deeper front and rear valances filled with dazzling chrome grilles, side sill extensions, chromed exhausts and 19in alloy wheels.What’s it like? It was always a beguiling car, this lid-less Maser, and after these additions, it has become just about the prettiest new convertible money can buy. But then we’ve never had a problem with the way the Spyder looks. Most of the criticism Autocar has made of the Spyder has been aimed at poor refinement, poor practicality and, ultimately, insufficient dynamic polish. It’s a shame to have to report that all three criticisms still apply.With the roof in place, there’s more wind noise at motorway speed than you’d find in, say, a Porsche 911 convertible. You’d get much more luggage into the Porsche too, since there are no rear seats and only a modest boot in the Maserati – another factor that blights its effectiveness as a GT. And when you ride over a sharp expansion joint on the motorway, even in the softer of two suspension settings, you can see the fascia in front of you shudder and feel the steering wheel tremor in your fingertips in a way that just isn’t a problem in a cloth-topped 911 or Jaguar XK.The Spyder is a mightily stirring and fast car nonetheless. Sixty-two miles an hour comes up in less than five seconds and, unchecked by an electronic speed limiter, it goes on to 177mph. The steering is incredibly quick-witted – a little too quick, perhaps – and in sport mode it corners flatly, grips strongly, and accelerates through the upper rev range with incredible urgency. In fact, when the glorious Maranello-made engine is burbling deliciously in your ears, you’re drinking in the classy ambience of the tactile cabin, and there’s nothing but smooth, empty mountain asphalt to tear along, few cars make a more persuasive case for themselves.Should I buy one? Unfortunately for most of us, mountain-top strops are few and far between, and in most other conditions, several of the Gransport Spyder’s rivals perform better. Its shortcomings will surely be remedied with the next model, due next year, but for the moment, it is difficult to justify buying a Gransport Spyder over a Porsche 911 convertible or (we had to mention it) the new 420bhp topless Jaguar XKR.