There are some dangers associated with a movement that large.
I had my bimax surgery 3 years ago, had 11mm mandibular advancement on one side, 9mm on the other (asymmetry). During the movement process, the mental nerve was damaged. This nerve feeds sensations to the lower lip and chin. He either cut it, or put significant stress on the nerve, because now, 3 years later, I am completely numb in that area. Not only that, but I had a genio, and he placed it too far foward and low (as in sliding genio with lengthening), that I cannot speak correctly.
If nobody believes this opinion, you can search for "numb chin" syndrome.