Well I said I’m “enough of” an empiricist, not that I was denying what you lay out. But in the case of such thought experiments that can be begun with a small empirical circumstance such as a small group of individuals and then logically extended, a lot of the time I’d still want to say “I don’t believe that the logic is a simple as you suggest to extrapolate”. While I’m not in favour of radical skepticism, I find a healthy skepticism in the face experiments which could theoretically be run to test their validity - in this case the skepticism shows that “no, you can’t actually organise a China brain, ever, because we can’t corral 1.4 billion people or even get them on board with such an idea” so I’d find the logic faulty that says there’s a proof for functionalism regardless within the thought experiment.

A basic knowledge of today’s physics would say the hoverboard is not possible. An advanced knowledge of physics five-hundred years hence might show a way of doing it. It “wasn’t possible” to build a thinking, talking machine five hundred years ago and look at us now.