not sure if you mean ride height or ride quality?.
I replaced my springs with the pro kit after some oem spring fatigue breakages. The ride height lowering described by eibach (cannot remember off the top of my head) is relative to the a3 so as the s3 is already lowered compared to the a3 the lowering is very minimal on an s3.
I thought the ride was actually softer than stock when cruising but the springs are progressive so they tighten up in the corners and feel nice........no special tie arms or arbs were required.
I will however be upgrading my arbs with the eibach kit over the summer as i still think there is too much roll for my tastes.....but my arb bushes are shot so a timely upgrade for me.
in terms of immediate turn-in stiffness (if you want it), i think that is a shocks upgrade required .........i will be getting the koni shocks when the bank manager allows....
I have just done the suspension bushes inc dogbone mount (but minus arbs) with Superpro polys........huge improvement.....much tighter.