ACC is linked to the dampers, some springs may give a stiffer ride, some i have sampled actually ride better
A stiffer ride is not necessarily worse because in the terms you're talking ride is subjective.
There are many considerations but in general, if you lower the car you will have less suspension travel and in order to mitigate continually bouncing off the bump stops because of this reduced travel these lowering springs will normally be stiffer. Now you might find that by lowering the car (and re-aligning the suspension) that the car grips better (it may also handle better but handling's subjective) but in the real world where roads are not billiard table smooth you might find that the car actually generates less grip (and handles worse because you are struggling to keep the wheels in contact with the road)...
In the end, it depends why you are lowering the car. If you are doing it because you think it improves the look of the car that's great but remember you might make the car more uncomfortable and worse to drive.
If you are doing it to improve the grip/handling... then be careful... there is a very real chance that you will do the exact opposite. I'm not saying improvements cannot be made but what I am saying is that in trying to improve the car you might make it worse or even if you do find an improvement it is likely that this improvement will only be realised in certain circumstances.
