It would be highly unusual that anyone would need as much as 1.5 cms of vertical bone reduction of the chin or jawline. Most patients way overestimate how much bony change they actually need and where along the jawline they need it done. But for the sake of discussion, the limiting factor of jawline bone reduction is the location of the tooth roots and the course of the inferior alveolar nerve. Leaving a cuff of a few millimeters of bone, that would mean the maxiumum bone removal is closer to 1 cm except back at the jaw angles. Shaving that much bone would not be prudent in the chin area and a wedge reduction genioplasty would be a more prudent approach for prevention of a potential soft tissue sag.
You can decrease you chin height. Shaving the bone is a relatively inaccurate method and is not reversible. Performing a shortening genioplasty is more controlled and predictable.
Joe Niamtu, III DMD
Cosmetic Facial Surgery