I doubt you could get 20hp extra without changing the exhaust. The stock S8 has 400cpi cats, and baffled mid and rear box probably not optimal for performance.
So before re-mapping you would definitely need to change the exhaust and cats.
To get a even a bit more power you would probably need to port & polish the intake manifold and get some long tube headers made up (very expensive). Then a remap after all the physical changes. Have a look at JHM S4 videos on youtube, they got a lot of power out of their S4, it has more or less the same engine as the S8.
https://www.youtube.com/watch?v=3n7xVwjWWD0
The other way to 'power' is actually to reduce weight. Acceleration = Force (torque) / mass