Price and equipmentTHE Audi S8 is not affordable to begin with at $280,610 Plus on-road costs.

Adding the ‘Plus’ tag results in an equation of that base price Plus $70,000 – taking the sticker to $330,500 Plus options.

New equipment for the S8 Plus includes full leather dashboard ($14,900 on the regular model) and Alcantara headlining (typically $4050) inside, while a quad sports exhaust system, a different style of 21-inch alloy wheels, rear spoiler, carbon lower bumper panels and rear diffuser, and gloss-black trim highlights round out exterior alterations.

Optional kit featured on our test S8 Plus includes carbon brakes ($23,100), 1400-watt 19-speaker Bang and Olufsen audio ($14,500), night vision assistant ($5970), carbon mirrors ($3600) and spoiler ($2100), and double-glazed privacy glass ($2500) for an eye-watering $385,010 total.

More equipment could be added, which arguably should be standard, such as rear seat heating ($2350) with individually reclining back buckets ($5650).

InteriorAs expected the S8 Plus is a model of leather-lined indulgence inside, complete with wonderfully supportive heated and ventilated front buckets each with a massaging function, while the ‘carbon-fibre with red thread’ dashboard inlays are the only hints that this is a sporty model.

Interior issues are not the reserve of the S8 Plus, but rather the entire third-generation A8 range that arrived in 2010 and in some ways feels its six-year vintage. The steering wheel buttons are shared with the company’s A1 hatchback, for example, and newer products such as the sub-$100K A4 and TT include a ‘virtual dashboard’ that instantly dates this $300K-Plus S8 Plus.

The Audi Multi-Media Interface (MMI) buttons on the lower dashboard are a confusing mess in the S8 Plus and the main rotary dial lacks the touchpad function of even the RS6/RS7. Everything is well made, as expected, but compared with the grandiose Mercedes-Benz S-Class and tech-laden BMW 7 Series, this Audi does not feel special enough inside.

The high-mounted screen has been updated to offer high-resolution graphics and the latest connectivity including Wi-Fi hot-spot capability, Google Earth satellite imagery and even street view functions, however surprise and delight features – the night vision assistant that shows the way forward in black-and-white heat-soaked highlights, and the crisp audio system – are optional.

For such a large sedan, the boot is capacious but the rear cabin lacks legroom and beyond the climate control’s four separate zones and air vents mounted both on the lower console and B-pillars, delights such as screens, and reclining or heated seats, are either optional or unavailable.

Engine and transmissionThanks partially to its aluminium spaceframe construction, the S8 Plus ducks under a two-tonne kerb weight by 10 kilograms. Despite including ‘quattro’ all-wheel drive, the Audi is also only 20kg heavier than its newer Mercedes-AMG S63 rival that sends power to its back wheels.

Indeed because of quattro, the S8 Plus can place to the ground every one of the 445kW the 4.0-litre twin-turbocharged V8 petrol engine delivers from 6100rpm until 6800rpm, and all its mighty 700Nm made between 1750rpm and 6000rpm.

There is no need to check that, on overboost, the engine makes 750Nm between 2500rpm and 5500rpm, because a slight tug of the steering wheel will provide that information – yes, this Audi all-wheel-drive has slight torque steer, such is the amount going to the turning wheels even when divided between two halves.

No rival limousine can touch the claimed 3.8-second 0-100km/h of the S8 Plus.

The soon-to-be-replaced Porsche Panamera Turbo S matches that claim, but it starts at $445K and is not actually a limo, while the S63 costs $387K and is six-tenths slower.

Beyond the statistics, the way the velvety V8 delivers its torrent of torque through the impeccable eight-speed automatic is the definition of blending refinement with a sporty edge. At any time, and with the addition of as much throttle as required, the drivetrain steps up and briskly responds.

Back off the throttle and anything mechanical just gels into the background.

Through around-town running, onto the freeway and over hilly and twisty terrain, the resulting consumption of 13.5 litres per 100 kilometres was exceptional only one-quarter more than the 10.2L/100km combined claim.

Ride and handlingAs with its powertrain, around town the Audi S8 Plus is a silken performer, but on a country road it also does its mightiest to justify being called an RS8.

In either scenario, the adaptive suspension’s Comfort mode is too floaty, skimming over every minor imperfection in the road but also becoming quite wavy and nauseating for occupants. The middle Auto setting remains a superb blend of compliance and control, particularly for a vehicle wearing low-profile 21-inch tyres.

However, through corners either bumpy or smooth, the Dynamic suspension setting teamed with that mode for everything – steering, sport differential, engine sound – delivers amazing handling while never becoming too harsh like many sportscar’s firmest settings do.

The steering becomes direct and immediate, the throttle sharper and the automatic primed for more aggressive downshifts, while the engine plays a deeper tune. The agility for such a large, heavy car is astounding, and the S8 Plus can be tickled right up to the edge of tyre grip before just starting to fray around the edges.

Right there, it pales alongside an RS6 or RS7, but anything beyond that point is best left to racetrack work anyway. The astounding ability of the surprisingly smooth ceramic brakes, teamed with the way the S8 Plus resists bodyroll, maximises grip at each corner then permits hard throttle quite early on corner exit, would reduce a rear-drive S63 to a mere dot in its rearview mirror.

Safety and servicingSix airbags (including dual front, front-side, and full-length curtain protection), ABS, switchable electronic stability control (ESC), lane departure warning with lane-keep assistance, pre-collision warning with autonomous emergency braking (AEB) and a blind-spot monitor are standard.

ANCAP has not tested the Audi S8 Plus.

Audi’s Service Plan at a cost of $1900 includes servicing to three years or 45,000km at an average of $633 per dealer check-up.

VerdictIt is perhaps ironic that the Audi S8 Plus does not feel like a $300K-Plus limousine until it is able to deploy its performance and dynamic characteristics on the open road.

At lower speeds around town its lack of rear legroom (for its size) or outstanding kit (until the options boxes are ticked), and dated cabin appointments relative to its opposition, are magnified.

Consider the leathery luxury on offer, however, in addition to the diverse capabilities of the S8 Plus whether cruising on a freeway and blocking the outside world or engaging a driver on the road less taken, and this Audi’s pricetag starts to become justified.

For passenger-focused luxury, it is still best to see your Mercedes-Benz dealer, but at least until the new Panamera arrives, for the driver the S8 Plus (nee RS8) is king among plus-sized sedans.

RivalsMercedes-AMG S63 from $386,910 plus on-road costs>br>Special cabin and engine, without the all-weather dynamics.

Porsche Panamera Turbo S from $444,200 plus on-road costs
Epic engine and handling, but tight cabin and soon replaced.