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First the good news: if you happen to have a desert outside your back door, there’s probably not a better car to own in the world than this new £81,565 Porsche Cayenne Turbo S. After a day driving over sandscapes that would set Anthony Minghella drooling, I emerged not so much impressed by its abilities in such conditions as in awe of them.

Out here in Dubai, where the sand is as unrelieved as the vast blue sky, what you really need to keep going are the fattest, biggest tyres around, adjustable ride height to keep you from beaching yourself on the dunes and, of course, as much power as you can muster.

Driving in sand is not like driving in mud: if you creep along carefully you’ll be buried up to your axles while still within sight of the nice safe road you just turned off. As with any skill there are nuances, but your rule of thumb is to go like hell. There is nothing you would recognise as grip in deep sand, so the only thing that keeps you going is momentum, as defined by mass multiplied by speed. Which in the Cayenne Turbo S equates to 2,355kg times as fast as you damn well like.

With 521bhp it is the most powerful SUV on sale — by comparison the supercharged Range Rover Sport’s output is on the puny side of 400bhp. Porsche says the extra 71bhp it boasts over the standard £70,870 Cayenne Turbo has been achieved without needing to touch the engine internals but simply by raising turbo boost pressure, increasing intercooler efficiency and reprogramming the engine management.

But, being Porsche, simply reducing the Turbo’s 0-62mph from 5.6sec to 5.2sec and raising its top speed from 165mph to 168mph would never do on its own. The suspension has been beefed up, its electronic air springs and dampers reprogrammed and the brakes made bigger still. Visually, though, all you’ll see is new 20in wheels, round rather than square exhausts and a badge on the back.

On the straight and deserted roads of Dubai it feels imperious. It has so much power that, despite its hideous weight and an automatic gearbox, it accelerates faster than Porsche’s latest sports car marvel, the Cayman S. You sit so high yet are accelerated so fast it felt more like taking off in the Boeing that brought us here than anything related to the real car world.

Which brings us to the bad news. Most of us don’t live on the edge of a desert, or enjoy endlessly straight, deserted roads. Most of us live in cities and towns where a 521bhp SUV makes as much sense as a powerboat. I’m not going to wheel out all the old anti-SUV arguments but I remain staggered that people are prepared to spend so much on a car and then use it in the environment that suits it the least — the city.

It’s not even as if the Cayenne is good looking. For this money there are any number of conventional cars I’d rather own or look at. This, after all, is a car that costs over £23,000 more than its closest rival, the Range Rover Sport.

If you need to transport your family at speed, a BMW M5 will do it better and more quickly than this Cayenne, yet costs over £18,000 less. And if you need four-wheel drive, an SUV-type body and massive torque because, say, you are towing horses, would you not be better off buying a Volkswagen Touareg with a magnificent 5 litre V10 diesel engine? It has the same platform as the Cayenne (with a dramatically prettier body), uses a lot less fuel and is about £28,000 cheaper.

I enjoyed myself hugely in the Cayenne Turbo S but that was because Porsche took me to one of the few places on earth where its prodigious talents could be properly exploited. If you really need a Porsche that badly and can afford this Cayenne, go to your VW dealer and haggle over a V10 Touareg. Drive a hard enough bargain and you’ll get a Boxster with the change.

VITAL STATISTICS

Model Porsche Cayenne Turbo S
Engine type 4511cc V8
Power/Torque 521bhp @ 5500rpm / 531 lb ft @ 2750rpm
Transmission Six-speed automatic
Fuel/CO2 18.0mpg (combined cycle) / 378g/km
Performance 0-62mph: 5.2sec / Top speed: 168mph
Price £81,565
Verdict The world’s fastest SUV, if that’s what you really want
Rating 2/5
Date of release February 28

OR FOR THE SAME MONEY

Model Range Rover Sport Supercharged £58,000
For Superb off-road ability, great cabin
Against Slower and thirstier than it should be

Model Volkswagen Touareg 5.0 V10 TDI £53,640
For Magnificent diesel engine, stylish and well built
Against Lacks Porsche’s image, cumbersome on winding roads