For example, I currently live in Germany. Germany is currently importing boatloads of Latino and South East Asian STEM workers. Now if you look at the structure of German companies two things stand out:
1. there is a huge disparity between management and workers and this is reflected also in the salaries 2. all management are German (99.9% ethnic with maybe one 3rd generation ethnic Slav/Turk)
This situation is very likely true for most of Europe, except for maybe the UK.
So of course migrants in the US are more productive. In most countries immigrants are imported to fill a niche and the glass ceiling is held firmly just above that niche. And let's not even talk about entrepreneurial opportunities for immigrants.