I just measured the SSD thickness of a Samsung 850 EVO and it is 7mm. The Samsung spec sheet didn't have that info, it just said it was a 2.5" form factor. BTW, I also removed the DVD drive and installed another SSD as well. OWC has a good drive adapter to do this, called DataDoubler.
I have the MacBook Pro mid-2012 15" and I have installed Samsung SSD 840 EVO 2.5" form factor drive and it fits fine. Really makes the MacBook fast.