A little late to the party, but I have issues with the seats as well. Coming from a 2012 Volvo S60, which had the most comfortable seats I had the pleasure to sit in, these a bit of a let down. Initially, on the extended test drive, they felt great (I wish I saved that position), but I have noticed they put pressure on the outside middle of my back (lower lats). After a bit of investigation, I think the seats are a bit narrow for me. I compared them to my 2013 VW CC, which are very comfy and quite supportive, and noticed they are about an inch and a half narrower between the bolsters. I fit perfectly between the CC's bolsters, with less than half an inch lateral movement. So my back is resting on top of the bolsters in the MKZ.
Since this is my wifes daily driver, I only drive it on weekends, random errands, and family drives. Every time I drive, I slightly change the seat position and save it to memory. I've been continually tweaking and have found a spot which seems to have improved the situation. The true test will be our 9+ hour drive to Maine this summer. I'll be driving the whole way.
Despite the seats, I love this car! Ours is Velvet Black with Terracotta/Ebony interior. The panoramic roof is fantastic!