at the time the S8 was made it was a fast sedan but you have to remmember that was a few years back the S8 does 0-60 in 6.5 seconds while the the GS400 does it in 5.7seconds, but there is a catch to it the S8 was tuned out to be a grand tourer so it has audi's awsome quatro system and a more stiffend up suspension, what Im trying to say is what it lacks in speed it makes up in cornering, Ive also heard that Audi has an RS8 that is only sold in Germany that puts out 460hp thats M5 teratory right there, they also have that amazing little station wagon the RS4 can you say 0-60 in 4.5 seconds 
