i have to say i'd rather i was running a set of koni/billies and eibach springs than a £200 set of coilovers
and yes if you lower it much more than about 30-40mm the handling will get worse, it may feel like "its on rails" but the actual limit of grip will be reduced as the suspension geometry will be away to ****.
I find that most people with modified cars that are lowered seem to think they handle really well. "on rails" is a common description. I find however that this generally just means the car has no body roll. My brother had a nova on coilovers which felt similar as it was very hard, but if you threw it round a roundabout that was slightly uneven it would start to understeer badly as the car was actually bouncing off the ground over the bumps. My astra 1.4 on standard suspension with decent tyres (uniroyal rainsports) could go round faster, albeit it felt ****, and was leaning over on the door handles, but it still went round quicker.
I would bet that a stock S4/A4Sport suspension setup would handle better than something that was on its ring and really stiff, simply because the geometry would be correct and the springs and dampers would actually be working to keep the car on the road.