How long have you had those S-O3's ?; only I had S-O2's on the front of my 156. Grip was fantastic for the first few thousand miles, but then I found the inside edge going bald. I felt gutted - at over £100 a tyre - and only lasting 4 thou miles. Had to swap them onto the rear.
I now use budget Kumho ECSTA Supra 712. They have excellent grip ....and no receding inside edges as yet!!