The amount any given dog does best on will vary wildly. Older dogs actually need slightly more protein than the average middle-aged adult dog (Purina did a study about this a while back, the thought that they need less has definitively been proven false). That said, what constitutes "more" is up for debate, as very high protein kibbles are becoming more the norm, so its hard to say if "more" means 26-28% ("more" than the old standard of 22-24%) or "more" being 34-38% ("more" than foods like ToTW or many other grain frees which average around 30-32%).
Either way, I like Holistic's options, you have a 28% grain free fish, and a 32% grain free turkey. Either, in my opinion, would be ok to try.
GSDs- Mulder, Ridley, Vega
GSD on the half shell- Rigel