The S8 is incredibly fast for a car of this size. MotorTrend last week got an 11.8@118 out of it in the 1/4 mile with 0-60 in 3.5 seconds thanks to the all wheel drive. The B7 also has all wheel drive as an option but MotorTrend is testing the rear wheel drive version of the car.
This is the updated 540 horsepower version of the Alpina B7 with the N63tu V8. So on paper it has more than the Audi S8. It isn't faster in a straightline however as it runs the 1/4 mile in email@example.com. THis is five MPH short and 8/10's short of the 11.8@118 the Audi S8 manages despite its horsepower and torque disadvantage on paper, incredible.
The cars were dyno'd at the wheels which is something great to see in a comparison video, well done MotorTrend. The S8 put down 479 horsepower and 422 pound-feet of torque to all four wheels. The S8 seems to be underrated quite a bit. The Alpina only managed 414 wheel horsepower and 437 pound-feet of torque making for a car that is overrated on paper. This explains the acceleration difference which clearly is not just due to all wheel drive helping put the power down.
MotorTrend really likes the S8 more. They said the B7 has poor throttle response and also that the car feels big and heavy yet doesn't hide it anywhere near as well. The downsides on the S8 are the steering (traditional Audi weak spot) and the brake pedal feel.
That said, MotorTrend feels the S8 is utterly fantastic. Audi has raised their game while BMW seems to have dropped the ball. What is going on?