Okay, but I would expect the main takeaway is that, for the purposes of things like tachyons and black holes and CTCs, the "speed of light" is still c, not wha

Okay, but I would expect the main takeaway is that, for the purposes of things like tachyons and black holes and CTCs, the "speed of light" is still c, not whatever speed one measures in various non-vacuums. The only assertion of which I'm aware that the apparent speed of light in a medium is what really matters for relativity was Ron Mallet's work: https://www.phys.uconn.edu/~mallett/main/papers.htm https://www.phys.uconn.edu/~mallett/main/papers.htm . I don't believe there's been any experimental confirmation of it, though.