best protein percentage?

Every breed and every dog is different. 26% is good if the protein is coming from meat and not corn. I'd want to make sure my money was going to the best quality food and not the highest protein. I'm not downing anything here, I'm not familiar with the brands of which you speak. If the dog is highly active and being field work there may be some advantage to it but I'd seek the advice of professionals.

I did check those brands on http://dogfoodadvisor.com/ and the Diamond Naturals checks out pretty good 4 out of 5 but they don't review the chicken or extreme athlete.

Seeing that your dog is a working type dog, and he is extremely active, then you can up the protein level of his food. There are some that are as high as 45%. Most foods are around 23-26%, which is great for smaller inactive dogs. Too much protein can cause liver and kidney problems. Taste of the Wild and Orejin are 2 foods that are quite high in protein, if you want to research them.

Extreme Athlete is also higher in fat.

I like the ingredients.....the first two are meat, and no corn or soy. My two big dogs are doing well on it.

One is 100 lbs and the other is (a more active) 80 lbs. They both stay a nice weight on only 2 1/2 cups of food a day. I had to feed them something like 4 cups a day of the regular stuff, and their body shape was different.

I really see a difference in body shape with the higher meat foods. Someone gave me a bag of Wellness limited ingredient food, which I gave to my food-sensitive chi. She dropped fat, tucked up her belly more, and developed more muscle, although she was in pretty good condition before.

That is a change I see regularly when I put foster dogs on better food.

It wouldn't hurt at all to go with a food with a higher protein content. Keep in mind that you may need to modify his portions after the switch, feeding less since the new food is more calorie dense (I see from the Diamond website that the Extreme Athlete formula has 4,710 kcal/kg compared to 3,708 kcal/kg in the Chicken formula) and labs do tend to put on weight easily. :)

This would depend on how many calories he burns off in a day, which depends on how much exercise he gets. Working dogs like sled dogs, for example, can tolerate more protein than the average pet could. Reason being they need it for energy and they burn off a lot of it, taking pressure off their kidneys.

If you want to try it, go right ahead, he is your dog after all.

Most dogs do well on higher protein, so yes. Some dogs get loose stools even after a lengthy transition, because the food is too rich, but that is a minority.