Mixing would be a detriment to your handling balance. Michelin has specifically recommended against mixing PSS and PS2. This is due to the substantial change in grip.
Go with another pair of PS2 on the back, and by the time the fronts are gone you should be about ready for a set of 4.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.