I think the most responsible way to approach this is to think of the dog first, and your use for the dog second.
That is, the health and welfare of the dog is a more important consideration than any effect on its hunting performance, irrelevant of the owner's enthusiasm for the latter. Remember folks, hunting is something we do for fun. The vast majority of us do not depend on hunting success with dogs for our own survival.
An intact dog will be prone to more adverse health effects than a neutered (neuter is the technically correct term for both males and females)
Females:
Intactness predisposes the dog to mammary gland adenocarcinoma (that's breast cancer in dogs). To be most effective, the female should be spayed prior to the first estrus. That's right folks, not even one heat. If she has 3 or 4, there is almost no benefit in terms of preventing future adenocarcinomas. Besides this, there is also the potential for pyometra. That is the uterus filling with pus. This will kill your dog if not treated, might kill your dog depending on how far gone she is when you get her to surgery.
Beyond this is the constant risk for pregnancy and nuisance of pseudopregnancy.
Males
Keeping with the cancer theme, removal of the testicles absolutely prevents the possibility of future testicular malignancies (Sertoli cell tumors most common I believe). Also prevents prostatic hyperplasia (any of you guys in your 60s probably know about this). Neutering also dramatically reduces the probability of perianal adenomas/adenocarcinomas (those are tumors on his anus...nice), and perineal hernias (the muscles around the anus weaken, allowing pelvic contents and potentially even the bladder to pass through. A herniated bladder can be life threatening, whereas a rectal diverticulum will just make your dog permanently constipated (I hope you like rubber gloves and Vaseline). What's worse, perineal hernias are very challenging to repair surgically, many attempts simply fail. Besides all of this, a neutered male is much easier to have around the house.
So, the responsible thing to do is to neuter your animal. The "musculature" factor is a non-issue. If you want to increase your dog's stamina, my suggestion is keep his weight down and make sure he gets a lot of intense exercise (i.e. running long distance, not walking on a leash) every day. You increase it over the summer so he is ready to go for the fall.
Then again, I wouldn't want reason to get in the way of "hunting lore"
Fat