A surgeon whose practice is devoted almost fully to Asian plastic surgery is probably pretty good at it.
A surgeon whose practice is devoted almost fully to, for example, Asian rhinoplasty is not just a specialist but a "super-specialist." Most likely, he or she is very good at it.
At the other extreme, a plastic surgeon who performs an occasional operation on a patient of Asian descent is really just a "generalist" and not likely to be as accomplished as the others in this unusually specialized field of cosmetic surgery.
While it goes without saying that most cosmetic surgeons practicing in Asia perform most of their plastic surgery on Asians, this alone does not constitute a great advantage over the specialist doctor in the West. Numbers alone are but a single variable and not the most important indicator of surgical excellence.