An initial after action report:
Just installed a 155mm Power Mirror. sit bone width is 131 so this is what Specialized recommended.
It was hard to tell where the sit bones were positioned on a simple 15 mile ride simply because the seat cushioned them so well.
The seat cover held me in place very well. Maybe too well as it does not encourage movement on the seat.
Little peroneal feeling. Since you do contact the seat with that area you can feel it, but no pain. A longer ride might alter the result as might a different seat tilt.
No digging into my upper thigh which the regular Power Expert did.
All in all it looks quite promising. We will see after 60 miles or so.