When I bought my car (used) it had PS2's all around, I found the car very tail happy and it would step out on me even during mildly spirited street driving. I put PSS's on the rear, and have not been able to break the rear end loose with the same type of driving. Under hard turns, the front actually pushes now (with PS2's on the front). I have found the PSS's far superior to the PS2's, which I didn't think was possible. These tires are very sensitive to alignment, however. My car is beautifully dialed in with very minimal toe. You may have poor aligment which could be steering the car under load. Don't give up on them...they are fantastic tires that gip almost as good as R compound tires.
Also, are you running stock widths? I have found that the PS2's and the PSS's have soft sidewalls that need to be slightly stretched to have effective cornering stiffness. If they are bulgy (tire on too narrow of rim) then they feel mushy.