I definitely got significant value out of studying CS in a structured/academic context after being self-taught; it helped a lot of concepts that I already fuzzily understood gel together. For instance, I had written little config file parsers, and even most of my own (mini) programming language, but taking a course on programming languages, studying grammars, lexers, parsers, etc. helped make everything click and feel less shaky.

I'm not sure the diploma is a huge deal, as long as you have some industry experience (sounds like you do) and can demonstrate your skills. Having some references who can vouch for you is good to have, too.