Not quite sure whats going on there but something isn't right. I have a converted MK CS8 that I don't really use much but I have used and with basically the same payloads you run. There is slight efficiency drop but nothing close to what you are having. I have a new brushless gimbal for it so I will take it out soon and I'll report back, but I think you either are using the wrong mixer or wiring or something. This was it with a GH3 onboard. It runs somewhat similar with the 5D3 although a minute less flight time. http://forum.freeflysystems.com/index.php?threads/coaxial-cinestar.782/#post-11074
After replaying this incident over and over in my mind and poring over the data with a fine tooth comb, and having the Council Of Elders review all my info, I comfortable with the following analysis of my near disaster: I believe that one of my pairs of motors came loose on its boom, causing the motors to slightly rotate, or maybe rotate badly. This is something that no copter is capable of recovering from, but somehow the motors corrected themselves enough for me to land the copter with no damage.