Well, there are two major transhumanist organizations.

Depending on what you mean by "major", I suppose. My first Google hit for "transhumanism" is Humanity+, and though I know nothing about them they at least seem to be in the same category. There's also the much maligned Lifeboat Foundation, which could hypothetically count as a transhumanist organization. So there's two more after five minutes of Googling.

It seems that you routinely post declaring that you are skeptical of SI's claims.

That's certainly not my MO for participating in LW. I'm a mathematician, not a loud contrarian. I'm currently working on summarizing Pearl's work on causality so that people can make more sense out of the sequences, and therefore help more when Luke's polyethics project gets off the ground.

Unfortunately, I'm also studying for my prelims, and so progress on Pearl has been a bit slow.

Declaring yourself to be skeptical, without explaining what evidence you are looking for, just doesn't seem to contribute to the debate all that much.

As I explained earlier, any hypothetically available evidence would have to be judged relative to some metric, and it's not worthwhile to sit around and discuss which metrics are optimal, particularly when nobody is in an impartial position to do so.