If you want to go much faster, in reality, keep it stock and start saving your money up to change the car. a 4.2 S4 can be had for £5000 upwards now, there is no way you would get better bang for buck tuning yours to get similar power.
Alternatively a 1.8TQ would be much more tunable and worth ploughing some cash into.
I have had a 3.0 Quattro myself (saloon though) and put an exhaust system on it and a panel filter and a map, and it really isn't worth the money.