Hi, I had FFS with Dr O in April which included a sliding genioplasty. You can see some pictures by clicking on my web link on my profile (next to the email button). Height and width of my chin was diminished by 5mm each. No change was made to projection. I did not detect dimples on the side of the jaw which used to be common with sliding genioplasties, and so it appears that he has been refining his technique relative to say, 20 years ago. In your case, the geometry of your jaw will be affected differently as it seems you'd like to slide your chin outward. As you do so, your mento-labial fold will deepen. He can fill that with bone paste.
Hugs, Beatrice