Based on my experience with my dog, this isn't an unusual amount that he is feeding depending on the dog's age, weight, daily activities, and dog-specific needs (such as how they process food). Each dog is different. I used to be able to feed my dog about this much when he was young and he would barely gain weight. I moved him to a higher protein diet (BG) and can now feed much less (about 4 1/2 cups a day) during hunting season. This keeps him at a steady weight (around 70 lbs). My particular dog has always had stomach issues and it took a long time to find the right food and food amounts. I also use Purina's Probiotic to control his gas and stool firmness. It took a long time, but I finally figured out what works for my dog. Don't let others tell you what works or doesn't work for your specific dog. A higher quality food may make it easier for you to feed less--it did for me.
As for feeding before, during, and after hunting--I've heard it all. Most everyone can agree that feeding a lot within 2 hours of hunting a dog hard could cause torsion/bloat. I have a good friend whose dog got torsion without him screwing up this general rule, so go figure. A little during the hunt may help with energy amounts, especially if you have a dog that can become hypo/hyper glycemic--I always forget which is which. Regardless, some dogs need the sugar more than others. I personally feed before the hunt (at least 1-2 hours) but half the amount I normally do int he morning, and then the remainder at least 2 hours after he is done hunting. Depending on the amount of energy expenditure I also feed small treats throughout the hunt (bananas, granola bars, a little meat from my sandwich). I've had vets tell me to give them karo syrup during the hunt to keep. Again, this all can be dog-by-dog basis. Try different things, within reason and never feed when the dog's heart rate is elevated or just about to be elevated (hard work) to avoid bloat/torsion. I've also heard folks say that food that expands more when wetted can cause more of a problem than others, but who knows for sure.
Good luck finding what works.
"We shall never achieve harmony with land, any more than we shall achieve absolute justice or liberty for people. In these higher aspirations, the important thing is not to achieve but to strive."