Thomas is extremely conservative, far out of the mainstream of American legal thought.

As are Rehnquist, Scalia, and others, yet Thomas is by far the least respected, least cited, and least noted. (Of all the people you had to cite as accomplished black people, you had to pick Clarence Thomas...) If Thomas is still so 'extreme' despite his bully pulpit for expounding & enforcing his views, that also says a lot about how good a justice he has been.

That said, worse Supreme Court Justice is roughly like worst tenured physicist at MIT.

Not really. The tenure process at MIT can be trusted a lot more than 'whatever candidate is young, politically expedient, and can be pushed through Congress', especially considering that justice nominations used to be relatively deferential. A candidate has to be as bad as, say, Harriet Miers to get rejected. I don't know how many physicists would make tenure with a similar slate of accomplishments and, to illustrate the relative looseness, highly public accusations of sexual harassment.

In particular, not-talking-during-oral-argument doesn't mean not involved

That would be reasonable to note if he wrote many majority opinions, played a large role in changing the others' opinions (of course, then he'd look less 'extreme' if he was contributing behind the scenes, wouldn't he?), or in any way was not a ghost who could be replaced by his clerks with no one the wiser.

Your reference to his position on affirmative action is particularly confusing to me, because his point is a standard, fairly mainstream argument against AA.

You misunderstood my point. My point there was that I agree with the criticisms of AA as harmful and pernicious through its effects in promoting people to positions and degrees they are not fully qualified for (the relevance of which should be obvious), and I was noting that far from being a nasty personal attack by me on Thomas, he himself says it played a part in his career, and who are we to disagree?

I can only point to my professional work as an attorney for special education students to give you a sense of my experience with what an 80 IQ student is like.

That's not an answer to any of my other questions. Why do you think your limited, bubble-filled experience is good evidence for overriding a century of carefully constructed tests drawing on millions of nationally representative people and exhaustively vetted for bias as documented in books like Jensen's Bias in Mental Testing?

(Even if we granted your special ed beliefs accurate status, although I don't know about that either - I too was a special ed kid, but any lawyer who spent some time with me as my family fought the school district would not have had a representative impression of special ed kids, both because I was unrepresentative (and that's why I was mainstreamed), and because the long cumulative day to day interactions are different from occasional interactions. My mom still works with special ed kids and mentioned that one of her teachers had her nose broken by one of her kids who was handleable right up until he broke her nose; one of my best friends was also in special ed, and he could be a pretty decent guy for weeks or months at a time until his anger problems finally exploded at you - we drifted apart so I'm not sure what happened to him but last I heard he was in prison, which did not surprise me in the least bit. Life is as much about the lowest points as the average points.)