A roll on race will simply favor the S65 AMG but it is surprising how well the stock Audi S8 does against it. The S65 AMG is heavier at 5097 pounds so that is a disadvantage but the car is just a monster up top. Eurocharged is able to get another 79 wheel horsepower and 100 pound-feet at the wheels out of the car with its tune. It isn't losing from a roll to the stock S8, period. It also should not lose from a stop but it looks like some driver mods are necessary.
The CLS63 AMG with the Performance Package also does well. It does lose from a stop to the S8 (as expected versus the all wheel dive Audi) but up top from a roll it also pulls the S8 relatively easily. The Performance Package M157 is simply stronger.
The Audi S8 would benefit from a tune and also from a better driver. The S8 owner does not seem to know how to brake boost or be in the right gear. He simply seems to get on the accelerator and then let the transmission adjust once the race begins. Regardless, the S8 is a stout car car these two benzes are faster. It would have been nice to see a race just between the S65 AMG and CLS63 AMG but we can't always have everything we want. Videos are below.