That may be true, but the school setting and that social cultural knowledge ABOUT the Ivies plays a role. I grew up in rural South Texas in a town of about 6000 people. No one I knew went to an Ivy and they seemed VERY far away. Even with the academic ability to get in and succeed there, you need a certain amount of social and cultural capital to know how to go about it and even see it as a possibility. Now, most of the kids from these schools probably already have this from their families, but attending an exam school just adds to that social capital because the Ivies are already actively looking for students from these schools.
If you lived in an urban area on the East Coast in a middle to upper middle class family, a 140+ IQ may be all it takes and an exam school can only enhance those opportunities. But without those social benefits, one's intellectual ability and academic success can easily go unnoticed and unsupported. To me, the representation within the Ivies and the findings of the study that these students do not perform significantly better (methodological questions notwithstanding) suggest that any "value added" by these schools is social/cultural not academic.
And then of course, there's the money...
