Protein levels have absolutely nothing to do with bone growth. Bone growth in terms of food involves calcium/phos levels related to how much you are feeding per gram.
Find foods that have a good cal to phos ratio and it shouldn't matter what the levels of protein or fat are (provided your dog doesn't have a sensitive stomach and can handle it).
GSDs- Mulder, Ridley, Vega
GSD on the half shell- Rigel