In the upper echelon of universities, is it the case that international students generally perform better than local ones?
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.