Most guys are running the same front and back....
I think on RWD cars such as mercs, M3s etc, the rear tyres are a lot wider as they;re what drive the car forward.
I doubt it would do anything on the A3 TBH.
Just go for good tyres (I got Conti Sport Contact 3 - Best tyres i've had!) and with the right tyre pressures, you should be able to minimise that oversteer you seem to get free with front wheel cars