Hey Gary, what kind of tuning are you thinking?
A good place to start what be a to remap the ecu. This would give you a health power increase and should give you a smoother torque curve. Each remap should be tailored to the car as every car is different due to age, mileage and the way it's been driven.
Just remember to have the car in fine fettel because the remap will just speed up the wearing process on parts that are already starting to go.
Other options could be to put a stainless steel exhaust on the s5 which will give you a better engine note plus each silencer you remove improves the way the exhaust gasses leave the vehicle, this can increase power slightly but we're talking 5-10bhp at best. People mainly do it to improve the sound ;)
obviously you can change the turbos, camps etc etc but then your talking serious money!
Hope iv helped cheers
steve