For now, though, all thoughts of outright speed are forgotten, for we're up to our hubs in the treacherous, shifting sands of a 140-square-mile area of Dubai that's dedicated to off-road desert driving. It's a daunting, alien and breathtakingly beautiful landscape, but one that has an ugly habit of swallowing SUVs whole should they stray from the path plotted by our expert desert-driving guides.

Following the instructions of our co-pilot to the letter, the Cayenne makes supreme progress, riding the dunes like a surfer, using the 4.5-litre biturbo V8 and Porsche Traction Management (PTM) all-wheel drive system to bludgeon us up each dune, then using gravity to gain even more momentum on the plunge back down. The drive and grip that the electronically controlled diffs manage to find force you to recalibrate your senses, as does the stability it displays when traversing perilously steep inclines. I lose track of how many times my subconscious screams that we're about to bog-down or simply barrel-roll down a dune, but every time the Cayenne taunts my timidity.

Even when presented with a seemingly insurmountable dune, or clumsily driven into the softest sand by yours truly, the Cayenne simply grits its teeth and pushes onwards, grinding its way through the sand with Olympian determination, even when it's up to its sills and forward motion has dropped to walking pace. Like the Terminator, it absolutely will not stop.

Of course, such conditions are extreme in the, er, extreme, but it's a graphic demonstration of the mighty Porsche's all-terrain capabilities. On road, it's the familiar Cayenne Turbo story of immense, physics-defying straight-line acceleration coupled to genuine agility through the corners. The brakes are also well up to the job of slowing this 2.4-ton monster, having been enlarged over the standard Turbo's from 350mm to 380mm at the front and from 330mm to 358mm at the rear. They are also 10 per cent better at keeping themselves cool, which is comforting in 36-degree desert heat.

Backed-up by standard PASM air-suspension (which gives six different ride-height settings and automatic self-levelling), standard 20in SportTechno wheels and 275/40 tyres with pressure monitors, the Cayenne Turbo S is a seriously rapid road car and, although clearly too big and heavy to genuinely thrive when driven hard, it expertly manages its own mass and momentum.

When set in Sport mode, the air-suspension is taut but fidgety; it remains sharp-edged over rippled or broken tarmac in Normal mode as well. Comfort mode softens the sharp edges, but you soon miss the added sense of control and response provided in Sport. It's evidence of the compromise needed to offer sporting dynamics in a tall and very heavy car, but so long as you're happy to toggle between the settings, the Turbo S's formidable chassis is effective and adaptable.

Dubai's road network is limited to say the least, with endless straights punctuated by unexpected 90-degree turns or race-track-like esses. A strange mix, then, especially when you're surrounded by an endless expanse of sand. And while downtown Dubai is rammed with traffic, once out into the desert there are hardly any other vehicles bar the occasional white Toyota Land Cruiser or the trucks heading to one of Dubai's countless construction sites.

Consequently, when we happen upon an arrow-straight ten-mile stretch, the temptation to test the Turbo S's top speed is irresistible. We drive its length in one direction to check for traffic, camel trains and, believe it or not, speed cameras, before turning to commence our run. There's a crosswind blowing in from the desert, and as we begin our run, sand flows across the black tarmac in snaking, fluid swirls. However, it's soon whipped into a sandstorm in the Cayenne's wake as we accelerate savagely and seamlessly, like a Merc SL55 on stilts, to an indicated 170mph. It's a fittingly spectacular way to end a surreal day's driving, and proof that the Cayenne flagship is the world's most potent production SUV.