SAT is an IQ correlate, and a part of the general "pedigree". In your field of law, if, say, you happened to interview a former clerk of, say, the late Chief Justice [the ultimate in post-graduate pedigree] - one could bet dollars against donuts that that person would have a stellar record, from SAT to law school and beyond, or he wouldn't have been there. And such people normally end in senior positions themselves.
What you are saying has some validity, but don't overstate it. A lot of what you are talking about has as much to do with networking among the top schools and its graduates, and legacies, as it has to academic pedigree.
My old firm wouldn't hire a former USSC Clerk based on clerking at the USC alone, or even largely. If they didn't fit our profile, for better or worse, we passed. That profile is fairly rigid for entry level, but a lot more fluid for laterals.
Doing well in schooling is circumstantial evidence of smarts, but not conclusive evidence of smarts. If all I knew about someone is that they scored in the top 1% of SAT scores, and all I knew about someone else is that they scored in the top 90% of SAT scores, I really can't draw a meaningful conclusion as to where they will each be a decade later, or two decades later. It just doesn't work that way.