@juillet - Your characterization of the research is just about completely incorrect. For instance, your understanding of the original stereotype research (the old Steele and Aronson paper) has been debunked many times. The disparity between blacks and whites did not in any way disappear when the task was recharacterized as non-diagnostic, even in the original paper. See, for instance, here: https://www.psychologytoday.com/us/blog/rabble-rouser/201512/is-stereotype-threat-overcooked-overstated-and-oversold. If you are interested in the subject, I urge you to read the original Steele and Aronson paper that started this whole line of inquiry, and you will see that the actual results have been typically mischaracterized in the popular press: http://users.nber.org/~sewp/events/2005.01.14/Bios+Links/Good-rec2-Steele_&_Aronson_95.pdf
As for your characterization that African Americans (and other lower testing groups) perform better than their tests indicate, actually the opposite is true. There is a large and deep literature on the “overprediction” of test scores for lower-testing groups: they actually perform worse than their testing would indicate. See, e.g., Rumor 2 here: https://files.eric.ed.gov/fulltext/ED562751.pdf. Again, if you start your reading on “overprediction” and “test bias,” you can spend months reading the research showing that minorities tend to perform even worse than their testing would indicate. This overprediction bias (which is actually a bias in favor of lower performing groups) has been noted literally for decades, and is one of the most accepted facts in the literature of psychometric measurement and personnel selection because it appears to be universal on tasks ranging from pure intelligence and academic testing, right through to workplace selection. If you are interested, Professor Linda Gottfredson is one of the foremost experts in the field; her work is very readable even to an interested amateur, and I would start my reading there.
Last, there is no question that affirmative action admits do worse in school than non-preference admits. (Note, importantly, that not all minority admits are race preference admits - some portion are fully qualified under normal admissions standards and they tend to do perform as any other student would.) For instance, the Arcidiacono study, often cited on CC, shows that blacks at Duke were at the 25th percentile by GPA throughout their four years, while white students were at the 60th percentile; in addition, blacks switched to easier majors as compared with other groups, especially after receiving poor grades in the sciences. Read the study here: http://public.econ.duke.edu/~psarcidi/grades_4.0.pdf. You will also see in that study that aside from lower SATs and entering GPA, black admits were lower, on average, on every metric that the admissions office employed to make admissions decisions, which included essays, letters of recommendation, “personal characteristics,” and course rigor (see Table 1, p. 7).
There is also extensive data available from the world of law schools demonstrating conclusively that affirmative action admits are towards the very bottom of their respective classes (over 50% were in the bottom 10% and over 70% in the bottom 20%). The seminal work here was done by Professor Sander in 2004; see, especially, Tables 5.1 (p. 427) and 5.3 (p. 431). The most recent data available (2017) show that nothing has changed over the intervening years.
These are just a few of the studies. I am not opposed to programs that help to increase the presence of underrepresented minorities on American campuses, but I do think we need to start being honest about the discussion. At some point, the claim that “racism” is holding certain groups down rings hollows, and is particularly vexing to the many people and institutions trying their level best to accommodate groups with inadequate preparation. My personal feeling is that colleges are doing a decent job, but professional schools are not - the disparities in ability at the professional school level (at least at law and medical schools) are so large and apparent that they are stigmatizing. (Professor Sander also does a nice job talking about the implications of this stigma for high level legal practice here: http://apps.americanbar.org/buslaw/newsletter/0060/materials/ch1.pdf.)