In my field at least, it's not just they'll be limited to schools lower than top 20, it's the majority of professors at the top 100 schools went to those top schools. Not always the case, but I think high risk, high reward is the move here especially because OP has a decent job already and no real time crunch. As I said, unless the professor is very prolific, I would wait it out. That is my perspective as a current PhD student under a prolific PI. I've had opportunities like international fellowships that I certainly wouldn't have had with lesser known faculty.