This particular example has had the full works thrown at it and was supplied new, fully converted, from the Hartge factory in Germany at a cost of ΂£82,246 (inc VAT). That's slightly cheaper than doing the work piecemeal, you'll be glad to hear. To convert an existing car costs ΂£30,500 (inc VAT) on top of the cost of the X5, which can easily top ΂£55,000 with all the options.

The car may sit on the largest wheels you've ever seen this side of a concept car, but overall it's a little lower than standard thanks to the 30mm lower sports suspension kit which includes rising-rate springs and the chunkiest anti-roll bars I've ever seen on a car - almost 25mm diameter up front. Amazingly, the wheels tuck into the standard wheelarches with only a subtle lip added along the top of the arch as part of the bodykit.

At the front, two enormous vents are cut into the Hartge front valance to help feed the new engine extra air. They look rather overdone to me, with just the standard radiator cowering behind them. Around the back, the rear valance is nicely smoothed off and painted body colour, together with the side sills which link the wheelarch extensions to give it a slinkier appearance overall.

Moving on to the mechanicals, the standard 4.4-litre V8 is enlarged and gently massaged with a longer-throw steel crank and gas-flowed heads with larger inlet valves and lumpier cams. The upshot is 345bhp at just 5200rpm and 375lb ft at 3550rpm (up from 287/5400 and 310/3600). You'd think the obvious engine to put in the X5 would surely be the 400bhp V8 from the M5, (which Hartge can tweak to nearer 460bhp), but unfortunately the transmission won't have it. Apparently anything over 350bhp and it starts to struggle. In fact the 5-litre Hartge conversion is detuned for the X5 (it usually makes 375bhp) so that its 'box and not so beefy axles stand a chance.

The first time you try a full power launch you'll get an idea of what they must go through, as the step-off is little short of shocking. It's bettered only by an Overfinch Range Rover, I'd guess, in the rarified world of tweaked 4x4s. Hartge quotes 6.3sec to 60 but that undersells it - the 0-30 time must be Porsche 911-rivalling in its tarmac-rippling intensity. As the tacho needle closes in on the red line, the aggression tails off, blunted by the stubborn, two-ton body, but the soundtrack that goes with the action is unforgettable.

The exhaust note dominates the cabin with a gorgeous, aggressive, old-school V8 rumble from tick-over to around 2500rpm before calming down somewhat as it stretches for the limiter. Thus it's nicely muted for motorway cruise speeds, making the exhaust a must-have accessory for X5 owners hankering for a little bit of character from their cars.

Overall, the increase in straight-line urge isn't that great, but then to make a two-ton car radically faster you need more than an extra 58bhp (a mere 29bhp per ton). The handling moves up a notch, though. The spring rates are nicely judged - at normal speeds you'd be hard pushed to spot the difference over the standard sport settings. Only when you push on do the lack of roll and crisper response at the wheel become evident. With those oversize wheels, it's a fine effort to make it this composed.

A lot of this has to do with the fact the standard offering is so well sorted in the first place. If it was my money I'd go for the Hartge exhaust and chassis options but the rest, well, the new X5 4.6S is due in the New Year and should offer most of the performance of Hartge's 5-litre for considerably less money. But if you can't wait I'd understand: this is the best modified off-roader I've ever driven. Just don't get a white one.