I believe insurance that covers top surgery covers it as transgender related surgeries. Unless you have severe back pain, I know people getting covered for that reason (usually you have to have a series of treatments and hx of it) some surgeons are sympathetic.
Yes, it is getting covered. For the most part, people are getting covered in large companies and/or liberal ones (like Apple or Google vs Hobby Lobby). I do know quite a number of people who are getting it covered but it not in any way common. It is getting covered more often. You go by not the company, but the policy. For instance, Blue Cross has policies covering it, and many more not covering it.
Gender dysphoria is not considered a mental illness. It's considered a psychological problem (like an adjustment disorder), why it's in the Diagnostic and Statistical manual used in psychiatry and psychology. Transition is considered the treatment for it. Transgender is not considered to be even a psychological issue (except I believe in childhood).
--Jay