In a large skillet cook sausage until brown. Drain fat. Set sausage aside. In the same skillet cook mushrooms, celery, onion, and proisciutto, if desired, in hot butter until tender but not brown. Stir in parsley, thyme, rosemary, and pepper; set aside.
In an extra-large mixing bowl combine the eggs and vegetable mixture. Add cornbread stuffing mix and herb-seasoned bread stuffing mix; toss until all ingredients are well-coated. Add the cooked sausage. Add broth and enough water to moisten as desired, tossing lightly to mix.
Use stuffing to stuff one 16-pound turkey. Place any remaining stuffing in a casserole. Bake, covered, in a 325 degree F oven for 40 to 45 minutes or until heated through. Makes 16 cups of stuffing (16 to 18 servings).