Aluminium spaceframe chassis, Quattro 4-wheel drive system, Tiptronic transmission..... these technology once gave Audi A8 the status as the most technological advanced luxurious sedan, but failed to win it the reputation as the best luxurious car in the world ( it's Mercedes S-class or BMW 7 Series ) nor the most sensible choice of luxurious car ( it's Lexus LS400 ). Arguably, Audi didn't use such precious technology very well. They allowed the Quattro system and the iron block of the V8 to offset the weight reduction by the revolutionary ASF chassis, so A8 was actually heavier than the Lexus and came close to 7-Series. Performance of the 4.2 flagship was limited by the 4-speed Tiptronic. Handling, despite of the 4-wheel-drive, was ruined by the insensitive steering. 

After 4 unsuccessful years, A8 received an update. Firstly, Audi gave up the intention to make it a driver's car like BMW. They softened the spring and dampers to seek more supple ride and comfort, in the price of inferior body control and understeer. Secondly, they upgraded the V8's to (finally) feature 5-valves head, as well as variable valve timing at the intake side and a 3-stage variable length induction manifold. At low speed, the manifold is 800mm long, reduce to 600mm at medium speed and 300mm at high speed. Of course, this help to obtain a flat torque curve. After these changes, power and torque output of the V8s were improved. The originally weak 3.7 version benefit the most, raising power and torque from 230hp / 232lbft to 260hp / 258lbft. The 4.2-litres version now has 310hp / 302lbft, up a little bit from 300hp / 295lbft. This is a small number, but the peak torque now occurs across 3000 to 4000rpm, thus real world performance improved a lot.  

As a result, the new A8 becomes very fast, faster than BMW 740i and even marginally eclipsed the Lexus LS400. Only Mercedes S500 could be faster. However, the revised suspensions do not encourage BMW-level of handling. 

Because a lot of modern equipments were added, such as satellite navigation and 6 air bags, weight reduction has to be applied to deeper level - the suspension links, damper mounting and brake calipers are made of aluminium alloy now. This keep the kerb weight roughly unchanged at 1750kg while unsprung weight is vastly reduced. The result is a very supple riding comfort. 

Another mechanical change is the adoption of drive-by-wire throttle. Why ? because the new A8 has equipped ESP stability control, whose computer will intervene throttle and brakes for correcting the direction of the car.  

In the end, the exterior and interior were facelifted. There are new grille, new door handle, new xenon headlamps, new bumpers and 18-inches five spoke wheels to distinguish it from the old A8, although they are not necessarily look better. Inside the cabin, you'll see an A6-like design, still finish in very high standard of quality. 
 

A8 with sport suspension package

So, the package stiffen the spring and dampers, wear the car with 18 inches wheels and 245/45ZR18 tyres. With these on board, you'll feel the car is greatly transformed. Body control is very good, understeer reduced at lot. Then you'll be eager to throw the car into corners at great speed and starting to enjoy the improved steering feel (also well weighted and without pronounced kickback), the abundance of grip and the powerful brakes which are already in the normal A8 but you've never got the interest to try. 

On the negative side, most people will feel the ride as too hard. 
 

The new S8

The "phase II" V8, no matter under the bonnet of A8 or S8, is characterised by 5-valve head, VVT at the intake valves and a variable induction manifold. For the latter, oddly, the S8 chose a simpler 2-stage design rather than the standard A8's 3-stage. The new engine's compression ratio, at 11.0:1, is lower than the old S8's 11.6:1 and is actually the same as the A8. For emission compliance ? very likely. 

Although power and torque are increased by 20 hp and 15 lbft respectively to 360 hp and 317 lbft, the new S8 is still inferior to main rivals : Jaguar XJR (370 hp, 387 lbft), BMW M5 (400hp, 369 lbft) and Mercedes E55 (354hp, 391 lbft). Torque, especially, made the difference. Although the engine is smooth and refined, it can't deliver the sense of explosion as the competitors. The 5-speed Tiptronic, which is likely to be a bigger seller than the 6-speed manual, can't help acceleration too. 0-60 mph takes 6.7 sec, at least a second slower than the E55 (also automatic), let alone the M5. 

Compare with the A8, it has ride height lowered by 20mm, plus bigger brake discs (with aluminium caliper). Braking power is typically strong, but handling is also in typical Audi style. In other words, capable rather than desirable. Not too involving.