I guess I don't understand your question DC. An osage back? What does that mean? An osage selfbow?
Are you asking us to compare a bamboo backed osage bow (BBO) to a bamboo backed yew (BBY)? Or to compare a BBO to an osage selfbow?
I once made 3 bows of the same d/r profile, same draw weight, same trapped cross section. One was a laminated maple and fiberglass longbow, the second was a BBO, and the third was a BBY. The Yew bow was lighter in physical weight and consistently outshot the other two by about 10 yards I think. I can't remember the distance exactly since it was a long time ago.
I never directly compared the exact same profile between an osage selfbow and a BBO. But I think they would be similar. I'd guess the BBO bow would be a little bit lighter in physical weight for it's draw weight. Like Pat said, it's the ease and variety of altering the side profile while gluing on the bamboo that makes them fun to build.