In the upper echelon of universities, is it the case that international students generally perform better than local ones? This question is aimed at undergraduate level, since postgraduate students are more mobile. By "upper echelon" I mean those that are, say, in the top 200 universities in the world (presumably there are lower-ranked universities that struggle to attract international students).

The idea is that most admission spots are reserved for local students for political reasons - it would be a political disaster if the country's best universities, especially if they are publicly-funded, teach more international students than local ones. So international students already have a higher bar to clear to be admitted. Therefore the international students perform better because they are better (e.g. they are the top 10% of their country's students, while local students are only the top 30%). Accordingly it should not be surprising, in fact it should be expected, that most of the best students of each cohort are international.

Is this the case in practice?
In the UK at least, this is not the case. Non-EU international students pay full fees so there is a strong push to accept as many of these students as possible. They still nominally meet the same minimum admissions requirements,  but very few home students come in at the minimum. Once at university, many struggle with the language and differences in learning expectations and perform more poorly.