The United States is ahead of the rest of them by a huge margin, and unfortunately this gap is increasing with every passing year.
I agree with most of your statement except this one. Their roster this year is not that far ahead of Serbia. If they ever lock horns later in the competition, I think Serbia will win.
The US team has almost achieved a superhuman status and it is difficult to imagine any other team being capable of giving them a run for their money, much less defeating them. I think that their last defeat was against Mexico during the qualifiers more than a year ago. But then, the team was devoid of most of the NBA players. If I remember correctly, it was Marcus Thornton who scored the maximum points for US in that game.
I agree that Serbia is the no.2 team. I would rather say that Serbia is the team that has the maximum chances of defeating the Americans in the tournament. The betting odds for Serbia are quite favorable (+500 to +600), and it won't be a bad idea to bet in their favor. I am not even considering the other teams, such as Australia and Greece.