Pruning isn't the only theory put forward to explain synaesthesia. There is also the theory that synaesthesia is the result of disinhibited feedback in the brain. I think this theory has nothing going for it these days. Studies have found structural differences between the brains of grapheme-colour synaesthetes and non-synaesthetes, so I consider the disinhibited feedback model mostly applicable to artificial drug-induced synaesthesia. I guess one could also theorize that the synaesthete brain ends up with more connections after pruning becuse it had a ridiculous amount of connectivity to begin with. Perhaps the opposite is also true of a pre-schizophrenic brain. Regardless of which theory one favours, I think there are good reasons to believe that synaesthesia might provide a clue or two about schizophrenia, as a condition that looks like the opposite.
So, are schizophrenia researchers all over synaesthetes like a rash, badgering them to volunteer as research participants or to fill in questionnaires and surveys? Nope. I'm a synaesthete and I've been very interested in the goings-on in the world of synesthesia research for many years now, and not once have I ever heard about research on synaesthesia or synaesthetes with the explicitly stated aim of gaining new understanding of schizophrenia. The present state of the art is that there are still some neurologists and doctors who have never heard of synaesthesia, and there is still a risk that doctors and psychiatrists can fail to understand that synaesthesia and psychosis/schizophrenia are different conditions, even though on a structural level these things look like opposites.
I would also expect that synaesthesia should also be of interest to dementia researchers. Is the synaesthete brain, with all of its extra connectivity, protected from any of the different types of brain-eroding dementia, or are synaesthetes more at risk for any age-related brain problems? Once again, I have never heard of any interest of this type from the world of medical research. Medical research is all about drugs. Drugs are where the money is. Patients can be convinced to experience placebo effects from drugs that have no useful function at all. Drug companies can't lose, and nothing is easier for a doctor to do than write out a prescription. Don't expect any genuine curiousity about what causes disease and death from this corrupt system.