Thanks for sharing the info, I am about to get a 512GB M4 for my early 2011 15" MBP; & am a bit concerned about your long shutdown time, is it normal?
It sure is not normal, but it also seems to vary. Whereas previously it would take that long most of the time, the computer now shuts down quite quick (less than 5 seconds) in most situations.
I have no idea what it is, but it seems to be gone now.