Forensic fellowship can potentially cause a very decent change in income. It is not guaranteed. It requires good marketing via multiple niches (civil, crim, WC, DBA, PI, Occ. IME, Testamentary Capacity, military, intoxication-related, capital, etc) . If you can invoice 600 hours a year (little over 10 hours a week) and do a part-time 10 hour a week $200/hr clinical job (total 20 hours a week), you could earn as much as a gastroenterologist at around 400k. Of course this won't be true if the gastroenterologist does expert witness work as well. Also, if you do a forensic fellowship and work full-time in a jail or state hospital, this won't apply as well.
Going back to the addictions, I have heard of some non-forensic addiction psychiatrists doing big tobacco expert witness cases. They do about a 2 a year at 60k each. Perhaps they do other addiction-related IMEs (impaired professionals, FAA, etc). If you are interested in this, maybe get ABIME training or there is another informal Forensic medicine certification with training. Basically, you could any type of psychiatric IME or case with the exception of criminal.
I am looking into getting the addiction medicine certification via the practice pathway. The MRO helps as well.