One thing's for certain: you couldn't wish for a better starting point, for the C4S coupe is one of our favourite cars. Slightly firmer, lower and wider than the standard Carrera 4, and with stronger brakes to boot, uncompromising GT3 aside it's the ultimate expression of normally-aspirated 911. And there's the rub. The C4S is a 911 designed to go that bit harder and faster, so why in God's name would you want to chop the roof off it?

Sales, that's why. Amazingly a third of all 911s built and sold these days are Cabrios, up from 23 per cent just five years ago. Adding the wide-body C4S to the Cabrio range is sure to boost that figure even more. Proof that Porsche's current customers are too young to remember the decidedly dodgy 'Turbo-look' Cabrio era of the late 1980s. Let's just hope the flat-nose look doesn't make a comeback.

While a soft-top is never going to have the stylistic flow and cohesion of a Coupe, Porsche has done a thorough job of integrating the folding roof into the 911's iconic shape. The rear end's sheet metal has been subjected to a subtle but total redesign in an effort to make the Cabrio's shape work with the roof both open and closed. It's a well-executed effort, but to me the C4S's broad tail looks too bulky, particularly with the roof stowed. I reckon the standard, slim-hipped C4 Cab looks sweeter, but that's a subjective gripe.

As you'd expect, there have been plenty of changes under the skin in an effort to retain structural rigidity for optimum handling, not to mention passive safety in the event of an accident. To this end the side sills and pillars are reinforced, resulting in the C4S being both torsionally and flexurally superior to any other 911 Cabrio model. The total weight penalty is 70kg compared with the C4S Coupe.

I'll admit there's something very cool about being able to lower the roof from the keyfob in just 20sec, cooler still when you can drop or raise the roof while driving (so long as you're not doing more than 30mph). The quality of engineering, fit and finish is very special indeed. As soft-tops go, this system is right up there with the best folding steel roofs from Mercedes-Benz in terms of sheer 'how-do-they-do that?' wonderment, if not in terms of noise suppression or vulnerability to the vandal's Stanley knife.

Climb into the C4S Cab with the roof up and you'll feel slightly claustrophobic, as the rear side windows and rear screen are smaller than the Coupe's, but in every other respect the interior is business as usual. Porsche chose the Austrian Tyrol as the launch venue, and with excellent roads winding through towering mountain passes, it's not hard to see the attraction. It's the perfect way of demonstrating the attraction of open-top driving, even to a sceptic like me.

Concentrate on driving, rather than straining your neck to take in the panoramic view, and it soon becomes clear the C4S Cabrio preserves much of the Coupe's dynamism. The surefooted feel and planted poise remain, with terrific levels of grip, traction and a deliciously neutral balance. The 3.6-litre flat-six engine pulls hard and sings loud, carousing its way to 7000rpm with a shiver-inducing yowl. The brakes (steel discs with four-pot callipers, rather than optional PCCM discs and six-potters) generate supreme levels of stopping power and feel. True there are certain occasions where you feel the structure shimmy ever so slightly through the steering wheel and seat of your pants, which does distract and detract from the driving experience, but these are pretty minor hiccoughs in an otherwise impressive performance.

All of which leads me, grudgingly, to admit that the C4S Cabrio is a sublime machine. Offering more involvement and excitement than a Merc SL500 and more structural integrity and dynamic polish than a Maserati 4200 Spyder, it's a brilliantly executed convertible.