Stan - this is a legitimate and interesting question. I don't know of good, representtive, quantitative data that's directly relevant.
However, I can share some experiences from teaching EA content that might be illuminating, and semi-relevant. I've taught my 'Psychology of Effective Altruism' course (syllabus here), four times at a large American state university where the students show a very broad range of cognitive ability. This is an upper-level undergraduate seminar restricted mostly to juniors and seniors. I'd estimate the IQ range of the students taking the course to be about 100-140, with a mean around 115.
In my experience, the vast majority of the students really struggle with central EA concepts and rationality concepts like scope-sensitivity, neglectedness, tractability, steelmanning, recognizing and avoiding cognitive biases, and decoupling in general.
I try very hard to find readings and videos that explain all of these concepts as simply and clearly as possible. Many students kinda sorta get some glimpses into what it's like to see the world through EA eyes. But very few of them can really master EA thinking to a level that would allow them to contribute significantly to the EA mission.
I would estimate that out of the 80 or so students who have taken my EA classes, only about 3-5 of them would really be competitive for EA research jobs, or good at doing EA public outreach. Most of those students probably have IQs above about 135. So this is mostly a matter of raw general intelligence (IQ), and partly a matter of personality traits such as Openness and Conscientiousness, and partly a matter of capacity for Aspy-style hyper-rationality and decoupling.
So, my impression from years of teaching EA to a wide distribution of students is that EA concepts are just intrinsically really, really difficult for ordinary human minds to understand, and that only a small percentage of people have the ability to really master them in an EA-useful way. So, cognitive elitism is mostly warranted for EA.
Having said that, I do think that EAs may under-estimate how many really bright people are out there in non-elitist institutions, jobs, and cities. The really elite universities are incredibly tiny in terms of student numbers. There might be more really smart people at large, high-quality state universities like U. Texas Austin (41,000 undergrads) or U. Michigan (33,000 undergrads) than there are at Harvard (7,000 undergrads) or Columbia (9,000 undergrads). Similar reasoning might apply in other countries. So, it would seem reasonable for EAs to consider broadening our search for EA-capable talent beyond super-elite institutions and 'cool' cities and tech careers, into other places where very smart people might be found.
This is a comment because it's not actually a justification for EA elitism.
There are some okay-ish ways to quantify where students interested in Effective Altruism might end up. If we assume that, for a student to be interested in effective altruism, they need to have independently pursued some kind of extracurricular activity involving a skill of the kind that Effective Altruism might discuss, we can look at where the top competitors for those kinds of extracurriculars are.
One thing to beware is confounding factors. People who would be good for EA might be too busy to participate in these activities (either because they have busy class schedules or are involved in research or because they work outside of school). People might also be doing activities because they are superficially impressive, which probably isn't a good sign for thinking in a very EA way.
Here are some brief summaries of where top competitors in different American extracurriculars come from:
Ethics Bowl (https://en.wikipedia.org/wiki/Intercollegiate_Ethics_Bowl)- no clear pattern among the universities
Bioethics Bowl (https://en.wikipedia.org/wiki/Bioethics_Bowl)- similar to above
National Debate Tournament (https://en.wikipedia.org/wiki/List_of_National_Debate_Tournament_winners)- often but by no means exclusively prestigious US schools, seems to lean towards private schools a bit also? but I'm just eyeballing it
US Universities Debating Championship (https://en.wikipedia.org/wiki/US_Universities_Debating_Championship)- Mostly Ivy-League or similarly prestigious schools
Putnam Exam (https://en.wikipedia.org/wiki/William_Lowell_Putnam_Mathematical_Competition)- Strongly dominated by MIT
College Model UN (https://bestdelegate.com/2022-2023-north-american-college-model-u-n-final-rankings-world-division/)- no clear pattern besides DC-based schools tending to do well
I'm sure other people can add more to this list.
If you think that Putnam results are a strong predictor of Effective Altruism, that could justify more elitism. Personally, I doubt that.