If Jack became the Face of Boe, could even sacrificing himself to save New New York finish him off completely? Also if he was Jack, when and why did he lose the ability to regenerate the rest of his body? He was also stated to be the last of his kind, so when and how did Jack change species? Are these questions even interesting or worth contemplating?
Well, there's immortal and then there's immortal. Since Jack aged, we can assume it's not absolute immortality ala: Ashildr.
Tardis Wiki calls Boe a member of Boekind. Did we hear this on the show? Did they mention a whole race of talking heads? If it was Jack, he could have ended up mutating after several thousand deaths, or maybe he was beheaded and the part that survived was his head, and again, mutation.
Otherwise it's just a funny coincidence.