Is my surgeon being too conservative with my sliding genioplasty? (Photo)

I have a good bite with no problem. My chin is weak and not long enough vertically. In my country, genioplasty is not common for purely aesthetic reasons and I've only found one surgeon that does it. He declared that he thought 5,5 mm horizontal movement and 3,5 mm vertical movement would be the most we could do. In my eyes that seems quite low. Could that really be enough? He does seem like a dr that usually does these operations for medical purposes. I want to have a strong jaw at the least.