I've seen a 14mm genio which was done to provide better horizontal and vertical projection it looked really good imo, the bony chin portion was then split into 2 with the gap being filled using bone grafts from the patient's rib (this was done to add width to the patient's face)
check it, I found the exact name of this procedure it's called:
Horizontal-T genioplastyHorizontal-T genioplasty--(a modified technique for the broad or asymmetrical chin).
A modified technique for genioplasty is described. A horizontal, single-slice of bone is sectioned from the inferior border of the chin, with preservation of the lingual soft tissue attachment, in the conventional manner. This is divided sagittally into three segments, which are not necessarily equal, and are then repositioned in the shape of a horizontal 'T'. To achieve this, the lateral segments are advanced and approximated in front of the central fragment which itself is also advanced. These are fixed in position and when desirable any voids may be filled with autogenous bone or hydroxyapatite. This simple technique augments the chin to an equal if not to a greater extent than the conventional 'double slide' genioplasty. It combines augmentation with narrowing and has the flexibility to reposition the chin. This technique is not advocated for routine use but more for the special case when augmentation is required for the patient with a broad, flat or asymmetrical chin. A presentation of four case histories will illustrate the application of this technique.
http://www.ncbi.nlm.nih.gov/pubmed/2207038if anyone is wondering who the surgeon was that did that
Horizontal-T genioplasty that I mentioned it in page 1, his name is Dr David A Koppel
http://www.davidkoppel.com/David_Koppel/Welcome.html