Pricey, due to be obsolete in 2017, largely irrelevant in our diesel-loving marketplace... yup, the rare Audi S8 plus, with its biturbo V8 petrol engine, doesn't seem to make an awful lot of sense. But then you experience its utterly madcap performance and all of its downsides are forgotten. It's an incredible way to say goodbye to the third-generation Audi A8 saloon.

In the metal

You'll need to be the mother of all Audi cognoscenti to spot the plus version of this car, compared to the regular S8. Black detailing on the radiator grille, front splitter, door mirrors, window surrounds and boot lip spoiler is the main giveaway, unless the owner of the car has painted it in the Floret Silver matt-effect paint that's exclusive to the plus, and then fitted it with the optional Sport exhaust that adds a pair of giant oval exit pipes instead of the quad finishers of an S-model Audi, which is what the S8 plus would wear otherwise. It's a very sedate car to look at on the outside and even 21-inch titanium-finish alloys hardly advertise what magnitude of firepower this four-door Audi is packing under the bonnet.

It's much the same story inside, because this is no different to a regular S8's cabin. Like any Audi cockpit, it's all beautifully screwed together and thoughtfully laid out, and it still looks pretty special in comparison to interiors from other manufacturers. But when other 2016 Audis are sporting Virtual Cockpits and climate controls integrated into the air vents, a pop-up MMI screen and analogue dials with a digital display sandwiched between them no longer look like they're at the very cutting edge. Still, it's roomy enough for four adults within and the plus comes with some additional toys over a regular S8, which we'll outline fully in the 'What you get for your Money' section below.

Driving it

Taking the 4.0-litre twin-turbo V8 petrol engine of the standard S8 (which isn't quite as powerful, at 520hp, as the same unit installed in a regular Audi RS 6, where it makes 560hp), Audi Sport has gone to town on the A8 by hiking up its maximums by 85hp and 50Nm. With 605hp and 750Nm, that makes the S8 plus exactly as powerful as the breathed-upon 'performance' version of the RS 6 - and it blesses it with figures that compare extremely favourably to the most potent iteration of the Audi R8 V10 plus, as it's 5hp down and yet 190Nm up on Audi's supercar flagship. Crikey!

Such thunderous power slashes 0.4 seconds off the S8's 0-100km/h time, leaving the plus capable of a scintillating 3.8-second benchmark sprint, while you can optionally have its 250km/h top speed limiter raised to 305km/h. Unfettered, we have no doubt the S8 plus would blast past 320km/h and on to a figure that probably doesn't bear thinking about, given it's supposed to be executive transport first and foremost, and not some form of ground-based strike missile instead.

It's therefore the straight-line performance that dominates the S8 plus driving experience, as - even in an era when hyper hatches surge from rest to three-figure velocity in around four seconds - the Audi feels ludicrously quick. Its quattro all-wheel drive traction and magnificent eight-speed Tiptronic automatic gearbox bless it with neck-snapping step-off acceleration, while that huge 750Nm midrange means it can switch from sedate cruising to enormous speeds in the blink of an eye (or the inopportune flex of a right foot). Overlaying the exertions of the 4.0 V8 is a lovely, creamy and rumbly soundtrack, but without the upgraded exhaust it's far more muted in voice than a 600hp-plus machine really ought to be. Perhaps it's worth ticking that options box if you're going to order an S8 plus, then.

So it's a bit of a pity that the Audi flagship is ultimately let down by overly-light, numb steering and air suspension that's biased towards comfort instead of handling acuity. However, work through these two minor foibles and you find a chassis of rare talent underpins the S8 plus. It'll grip hard and corner neutrally at higher speeds no matter what the weather or road conditions, while the body never gets totally out of hand on the air springs. It's not quite as sporty as anything with an RS badge, and indeed something like the Audi S3 Saloon will feel better tied down, but for a 5.15-metre long four-door that clocks in at 2,065kg (even with aluminium construction), the Audi drives far better than it really needs to.

And, of course, it functions beautifully as a cruiser. It's hushed, comfortable and refined whether it's doing urban, extra-urban or motorway work, and it's really easy to drive thanks to the over-assisted steering and the slick auto. However, a note of caution: in 400km of mixed-roads driving behind the wheel of the S8 plus, it struggled to achieve the 10.0 litres/100km official fuel consumption figure, turning in 17.8mpg (a shocking 15.9 litres/100km) overall. Also, it is - perhaps expectedly - in the highest band possible for tax, so running one on a shoestring is simply not going to be possible.

What you get for your money

At €167,500 on-the-road, the S8 plus is a hefty €33,230 more than the regular Audi S8. For that, as well as the power hikes and the subtle styling tweaks you get with the plus, buyers will benefit from soft-close doors, four-zone climate control, a Bose surround sound system and MMI navigation plus with MMI touch on top of the S8's standard equipment, as well as Matrix LED headlights on the outside. Is a near-€170,000 Audi saloon a good deal? Difficult to say, although it compares well to the €280,000 you'd need to spend on an equivalent Bentley Flying Spur...

Alternatives

Bentley Flying Spur V8: really, neither V8 Spur has the power to match the S8 - you'd need a W12, but they're way too expensive. The Bentley has better steering and a finer cabin, the Audi is quicker.

BMW 750Li xDrive: BMW hasn't, as yet, launched the M760Li xDrive (it's an M7 in all but name), so the 450hp/650Nm all-wheel drive 750Li is as close as you'll get. The S8 plus obliterates it.

Jaguar XJR: Jag has much smarter looks and a more modern cabin, and it's also better to drive than the S8. But again, it just doesn't have the pace to match the blistering Audi.

Summary

The Audi S8 plus is the idea of a Q-car, or sleeper, taken to the absolute lunatic fringe extreme. It appears to be innocuous enough and, driven gently, doesn't feel that much different to a TDI version of the A8. But if you dig into its vast reserves of power and torque, all kinds of chaos break loose and there are few cars in the world that will be quicker than the plus across ground come rain, wind or shine. Yes, it's expensive and yes, it's punitive to run, but if you've got the cash, investing it in one of these undercover agent Audis would be a very sensible thing to do. It's a smashing car in all respects.