I wasn't at the Manchester kickoff, nor have we built an overpass to the official specs, so I can't make a direct comment on the field. However, the real field is aluminum pipe, not steel as called out in the "simplified" drawings. I think the welded joints on the aluminum field should be a bit stronger (and stiffer) than the NPT connections on the steel pipes. The extra stiffness of the connections should carry over to a stiffer overpass in general. However, the long span of the overpass is still a long span, and the aluminum (as you know) is 1/3rd the stiffness of steel. So, I can only imagine that, yes, it'll probably be as bouncy as you're presently experiencing.
We've already taken some good shots on the overpass with our robot, and the ball certainly makes the thing shake like jello. We've also knocked our overpass over. I don't recommend doing that, ever.
Bengineer