Once you get all the sheeting on, you'll see the strength of the wing. I was just as skeptical with my Fazer wing, but it came out very strong.

As far as inducing a warp with the bottom sheeting, nothing to worry about. Once you've got that top sheeting glued on, that wing is going nowhere warp-wise. If it's straight now, it's gonna stay that way, and if it ain't, it's never gonna be. The sheeting from leading edge to spar is what lets you get away with 1/4" balsa spars. It provides a lot of strength.