I think that there are likely to be huge differences from school to school in terms of how much middle school affects high school placement. In some cases it will have a big influence, in others, not so much.
More importantly, I also think that this entire discussion seems to think of college admissions as the ultimate “end point”, which it’s not. People develop at different rates and times; the “top 20” schools select for kids who have developed early on, but there are plenty of opportunities for those who develop later. Life doesn’t stop with acceptance (or lack thereof) to a top 20 college. Barack Obama started at Occidental College, then transferred to Columbia and went on to Harvard Law School and greater glory. Just one of many, many examples. My sister was a late bloomer, didn’t get in to her top choices, started at a LAC and then transferred to Brown; she did her PhD at MIT and is a nationally recognized scientist in her field.
My older son has been in GT for 3 years, and been doing supplementary coursework through Johns Hopkins CTY and the Art of Problem Solving, and is several grades above level; he’s doing Algebra in 5th grade. Placement won’t be an issue for him, but as a high gifted kid he has plenty to work on in terms of social and emotional development. My younger son was a special needs adoption from China who spent 3 years in an orphanage, and had physical disabilities and developmental delay due to malnutrition. He’s just beginning to catch up to his school grade. But he has an indomitable spirit, the ability to overcome any adversity, and wonderful social skills. I’m just as confident that he’ll succeed (perhaps more so) than my older son, regardless of whether he gets into advanced middle/high school classes or a top 20 college.
There’s so much more to success in life than where you get in to college.