using the 19mm rear (r32) with standard front may cause the back end to step out easily on lift-off. the r32 front end is far too stiff for the front of the s3, even with the 19mm rear.
you will get worse understeer.
sensible options are:
16mm rear, 19mm front (factory)
19mm rear (r32 or neuspeed) and 20mm front (later tt225 type)- this MAY work but i havnt tried it
better still is neuspeed front and rear 19mm and 22 front - this combo leaves the rear 1.7 times stiffer relative to the front and the front 1.7x stiffer compared to the 15/19 factory setup.
the r32 rear is about 75+ from a stealer, where as the 16mm is only 40+. the neuspeed rear is about 120+ and the front is 150+ i think
neuspeed do a 24 or 25mm front which you definately dont want...
the neuspeed bars also come with stiffer silastic bushes that take up much quicker than the factory rubber - this mkes the steering much sharper. the bushes sometimes squeak - my new neuspeed ones sqeaked at the front until awesome rapacked them with copperslip but the rear ones were quiet.
i did the 16mm rear then changed to neuspeed front and rear and it gives a much better improvement than the the effect of lowered spring and koni dampers (although my suspension was only 22k miles old when i changed)