This is a tough prediction. Most U.S. fencers in the Olympics don't have current FIE rankings in the top 10 (with the exception of WS). Seth Kelsey is at #11 in ME, but the other fencers all range in ranking from #18 to #56 in their weapons. If all the individual events are DE tableaus of 37, then the final rankings between 17-32, 9-16 will just be determined by initial seeding. None of our fencers can do worse than #37, but only the lowest 10 seeds in each event have to fence off to get to 32s, so we won't end up with a high seed placing below 32. Chances are, our biggest "choke" measure will be a high-ranked fencer who has a bad day and bombs out in 32s or 16s. Because the WS fencers have by far the best U.S. current FIE rankings, I'll pick one of them. Now, I suspect all three of them will do very well - it's just that since they're the only U.S. fencers with current rankings in the top 10, they're the only ones who really could finish well below their current rankings. I mean, even if Kelsey bows out in 16s, he'd probably finish only a couple places below #11. If our lower-seeded fencers drop their bouts in 32s, they'll be placed between 17-32 depending upon their initial rankings, which won't end up being more than a few places below their rankings. If (the fencing gods forbid!) Mariel drops her round-of-16 bout, though, she'll end up placing maybe 10th - 6 places below her #4 ranking. Sada's just too strong to pick here, so I'll go with Mariel and 6 spots, despite my strong feeling that she'll probably do better than that. Peter's number is just very difficult to predict.
The question you didn't ask (Which U.S. fencer will have the biggest "shine" measure?) is easier and far more fun to think about. The main contenders here will be Kamara James (ranked at #34), Soren Thompson (#25), Dan Kellner (#48), Jon Tiomkin (#57), Ivan Lee (#19), and Keeth Smart (#25). Any of these fencers have the potential to place far above their current FIE rankings, but I'll pick Tiomkin, because we know he'll be at least 20 places above his #57 ranking, and if he wins a few bouts, he could be more than 50 places above that.

Of course, if you use only the relative rankings of fencers within the tournament (i.e., 1-37) rather than the FIE ranking numbers, this quantity gets reduced, and I'd pick probably Keeth to finish furthest above his initial ranking.
It would seem, due to the Olympics not just taking the top 37 fencers in each weapon, and due to the mechanism of final placement in lower rounds depending upon initial seeding, that it's much easier to place far above one's ranking at the Olympics than far below it.
Flame away.