I think it would be reasonable to claim that the larger the proportion of smart people in your population, the better. If that's true, than any number of below average intelligence people would be a surplus.
I get your point, but I think what your describing isn't particularly meaningful. By definition, nearly 50% of any population is categorically an undesirable surplus.
Yes, you could argue that. But a _nation_ could improve its proportions by restricting immigration to a subset of the _global_ population.
To a certain extent, this principle is applied today. But I imagine it could be scaled up to something akin to highly selective university admissions process (e.g. interviews, standardized testing, resume critique, references). If a country starts with sufficient desirability, this could result in a positive feedback loop -- an increasingly competent and productive society.
I agree with you there. Unfortunately, the greatest motivation to bring in someone outside the country is rarely for their exceptionalism, its usual about making/saving money. And at most institutions, its far more cost effecting to get individuals who are just sufficient and will accept a substantially lower compensation than their domestic peers. The individuals who are exceptional have loads of options/opportunities in their own homeland.