In particular the famous Benbow study [1] that established that even among the top percentile of the population there are massive differences: the top quarter percentile (99.75-99.99) was 2-3 times more likely to have authored academic research later in life, and about 1.5x more likely to have attended postgrad education, to have gone for a STEM degree, to have gone for a PhD than the bottom quarter of the top percent (99.00-99.24).
To start testing the validity of the claims in this paper a study needs to be performed where the strength of effect has to be considered, when comparing a similar one-percenter group and a larger ten-percenter group. My prediction is that the strength of effect would show only negligible differences, confirming my hunch that the special treatment[1] is what created the new subdivision in this new group.
More reading material on this topic would certainly be interesting, if you have anything in mind.
[1]: The special treatment in this paper being telling a kid "you are only a 7th grader, but you are as smart as a high schooler".
I am asking, because if I have to choose between my empirical observations (anecdotal as they are, given it is n=1 observers), and a single piece of research without followups, I do feel justified to stick to what I have seen myself. But I am open to be convinced otherwise.
One very important piece of insight I gained throughout that time is that after you use some exam to get the top 25% (ish) percent of students, the exam performance really does not matter: it does not correlate with scientific output or originality of work. If anything, I had to unlearn some of the skills that made me good at olympiads because they were severely limiting my creativity. At an olympiad you know there is a solution, while true research problems might be unsolvable and need to be approached differently.
TL;DR: Exams (selective or not, hard or not) are great at giving you the top 20% of students, but they are inherently terrible at telling you who among the top 20% will be a productive scientist or engineer.