If anything, the fact that the US has a lot of advantages for athletes (high incomes, nice facilities, etc.) means that grouping all those athletes together for the Olympics probably disadvantages individual American athletes, because there's more competition for Olympic qualification. If each US state competed separately in the Olympics, how many more medals do you suppose that would all have received in swimming, track and field, gymnastics, etc.?
Interesting question! This article says how many medals were won in Rio by people born in each state - http://www.wxyz.com/news/national/rio-olympics-medal-count-b.... It doesn't make a lot of sense to count how many medals were won by people living in each state since the facilities for some sports are focused in a single state. Way beyond me to figure out who didn't make the Olympics, whether they would have medalled, and what states they are from :)
You could approximate what would happen by summing the medals won by all EU countries, and comparing it with a hypothetical team composed of the top N (3?) EU athletes per event.
