We can't measure them on an individual level; especially not on immigration where people can pretend to care more for laws. In contrast, people cannot really pretend to be of higher IQ. We can however get estimates for groups of people or people who do not have to fear negative consequences for "wrong" answers.

Personally I'd assume that these differences are more cultural than genetic, so I don't think the argument for separation holds. But I can look up original HBD posts regarding this topic later.

Edit: First I found, HBD chick has a ton of these, I think. https://hbdchick.wordpress.com/2015/10/13/family-types-and-the-selection-for-nepotistic-altruism/