I had a 2008 S5 for 5 years, other than servicing and a new AC pressure sensor and regas it wanted nothing. was a very reliable car.
Servicing was around 250-350 depending what it needed that was at Audi too.
As with all the FSI V8's audi make they sometimes suffer from Carbon issues but i can't say mine ever did. Or it didnt show symptoms.
Mine was also remapped with the miltek exhaust. Sounded lovely the triptronic was good, newer ones have the dual clutch but you lose the V8.
Only really got rid as i needed 4 doors with a new baby. So went to the B8 RS4, same engine pretty much.
https://www.carbonangel.co.uk/imagedump/S5/finese/background800.jpg
With handling to can tell it's a heavy car but i never tracked mine or did anything silly. Felt nice on some spirited drives however.