For what it's worth, I agree with higher protein as well. My senior golden (almost 10) has started slowing down on our walks and just in general. I notice it takes him longer to get up and down, especially on colder days.
So I went to the dog food store and picked up a good quality Senior food for him. (Fromm) & a joint supplement.
But then after the fact, I started doing more research and ultimately asked opinions here as well. After much consideration, I opted to try the higher protein for him, so ended up going grain free because that was one way I could up the protein--plus he has some allergies, so it just made sense for us to try it.
He's been completely transitioned over to GF for about 3 weeks now and he is moving better, already. Granted, he is on joint supps too--but I honestly didn't see that much of a change with him just on those (and senior food) for a good month or so.
I'm not sure GF is a miracle cure for us--but so far, it surely isn't hurting. And my dog isn't eating nearly as much and his coat is even better (though I thought it was great before on regular Fromm 4 Star line!).
Best of luck to you and your dog. I have a real soft spot for goldens.