In which term efficient? In terms of maximum lift they definetly not. In terms of drag, they are as well worse, but the difference is small. You can have look at my posting on the page before.
I can you assure that a symmetric foil with incidence is not much different than a symmetric one without incidence, and that an asymmetric one (but not when two are used at the same time) is always better than a symmetric one, apart from the fact, that it is not symmetric...

Cheers,

Klaus