I have no idea but I suspect they are fairly represented but to expand upon your question of are many of our best scientist and engineers are foreign then the answer is no going purely by numbers.
80% of the graduate degrees awarded in the US go to Life Science and Health Care professionals which are dominated by US students. In engineering the numbers are somewhat skewed.
In theoretical engineering it is true that many of the best are foreign. However in applied engineering it is dominated by US Engineers as many top US engineers by pass graduate school to become Registered Professional Engineers. Becoming a Registered P.E. is as difficult as earning a Doctorate or PhD. It takes 3-4 years apprenticeship and to be licensed you need to pass a two day comprehensive examination that requires mastery of most fields of engineering from mechanical, civil, electrical, environmental, engineering economics, etc,. It's every bit as difficult as passing the Bar exam or the National Medical Boards Diplomat. It also pays, in general, substantially more than being a research engineer or academic engineer.
I remember when I was a research associate at the Ohio State University graduate graduate school of materials science and engineering (which is a weird story as I am not an engineer and my graduate education is in human biology) the department was dominated by Asian foreign students and a I disabused them of the notion that they were better than American engineering students when I explained that while they were in grad school earning their PhD most of their US counterparts were working on obtaining their P.E. License and that those folks would earn substantially more with a P.E. license than they would with a PhD. That was an eye opener to many of them.