Generally speaking, facial fat hides the shape of your chin and jaw bones. This is a good thing for those who don't get FFS because it makes the gender of your bones irrelevant. If these bones are male, you rather want them to get hidden, but if you get FFS you will probably want the changes to result as evident as possible. What I mean is facial fat will probably reduce the visible effects of your FFS, as FFS is mostly about reshaping bones underneath the fat.