Very little is actually unthinkable in the educated world -- but if you're going to voice opinions outside the Overton window you'd better voice them in terms of actual arguments.

That depends on how far you are outside the Overton window, and also in what direction. On some particularly charged topics where the respectable opinion is remote from reality (or at the very least lacking firm justification and open to serious doubt), people are aware that there are plausible-sounding arguments against the respectable opinion, but believe that this is just seductive propaganda by crackpots or villains that has been decidedly debunked by the respectable authorities. (Even though that's not the case, and the existing attempts at debunking are in fact severely flawed.) So even if you make a perfectly calm, logical, and scholarly argument against the respectable opinion, you'll just trigger people's alarms, without being able to make them listen.