I have owned my ST for 6 months and around 4 months ago, I had intercooler, Eibach Pro kit, Flowmaster Exhaust installed. Exactly what you have described above is the issues I have . Driving a normal road with no bumps , no big deal . If there are decent bumps or potholes , the car seams loose and bounces. Some rain it seams to drive like crap even with brand new Michelin . Glad I haven’t gone down the rabbit hole of throwing the ends , away bar at this issue . But I haven’t found many post regarding this so not sure if it specific to this lowering spring, will have the vehicle alignment checked by a different shop .