I note this is a discussion that's fairly political (http://wiki.lesswrong.com/wiki/Politics_is_the_Mind-Killer).

migration becomes a natural substitute (for wealth redistribution)

I think this is one statement I have difficultly with. If the country concerned doesn't have an objective to improve the life of people outside the culturally dominant group, its unlikely that migration can be reasonably equated with anything like wealth redistribution. The example of open but very unequal countries given is actually illustrative of that. I don't think such cases are exceptions to a rule, I think there's no particular correlation at all.

A number of related issues occur to me:

• Speed/volume of effective integration in a country, and its role in reducing cultural factors of inequality.

• Welfare / public services budgets and their role as migration incentives

• Brain drain as a poor strategy to reduce inequality, and the waste of certain vital skills when they're located in some rich countries.

• Earned things (eg. rights / citizenship) are generally more valued by people. Perhaps initial disadvantage aids integration.

• Visible inequality breeds resentment. Perhaps initial disadvantage hinders integration.

• Effectiveness of spending on migrants vs foreign aid as a method of improving lives.

• Circumstances in which people will work to "pass on" the assistance they receive and use their new advantages to work to assist others.

I don't think there's a discrete answer to your question exactly, other than "it depends", and I don't want to get into politics, so I'll leave it at that. Thanks for post.