Well, one problem with simply regressing IQ against income is that to an extent the gain is coming from what look like positional or zero-sum effects like displacing someone at an elite university

The numbers are higher if you don't control for education. Remember that on all the major theories of returns to education educated workers are more productive: learning (IQ helps to learn), ability bias (which is just saying that controlling for education understates direct IQ effects), and signaling. On signaling accounts the signals have to be honest enough that employers don't find it profitable to ignore the educational credentials.

Externalities are interesting, but hard to estimate (except for obvious individual-level things like net fiscal contribution, crime, etc), and probably not a primary focus for parents.

if sample sizes are growing exponentially

Yes. There are large samples scheduled to be online within 2 years that could give a 10x in sample size.

I'm not going to continue this thread any longer though, as many of the pieces should be addressed in a forthcoming paper with Nick Bostrom.