The '05 U-20 team that beat Messi and Argentina, and tied Holland had Feilhaber, Adu, and Spector. Plus guys like Kljestan, Wynne, and notably didn't have Charlie Davies. Adu was amazing in the tourney (he was just 15), and Feilhaber signed with Hamburg based on his performance.
Yes. U-20 give you more of an indication than U-17. Chile had a U-17 team in 1993 finish 3rd in the World Cup, out of all the "stars" from that team, not one made it big. The U-20 team that finished 3rd in the Canada World Cup a couple of years ago is the basis for the current national team and a lot of the players from that team are playing in Europe currently.
Many of these U-17's will be on the 2011 U-20 WC team along with some 18 y/o's (like Picault), and could be on the 2012 Olympic team if that's an U-21 tourney. I'd venture to say this current U-17 team if Renken and Lletget were included, would beat this year's U-20 team if they played today. Heck, could maybe even beat them without those two. Argentina didn't even qualify for the U-20 WC, but it's safe to say they'll be fine. Shouldn't put much stock into results in these tourneys, as I don't think that Austria team that beat us in the '07 Quarters will produce two players as good as Bradley or Jozy.