Preheat oven to 425, with rack in lower third. Rinse turkey inside and out, and pat dry with paper towels. Place on a rack set in a large roasting pan.
Blend 4 tablespoons butter with 2 teaspoons salt; season with pepper. Loosen skin of turkey at body-cavity end with your fingers; spread mixture under skin all over (reach as far back as possible).
Gently push 3 parsley sprigs under skin on each side of turkey breast (use a table knife to push them as far as possible, keeping leaves flat). Loosen neck skin; slide a parsley sprig on each side of top of breast. Repeat with sage leaves, placing 4 leaves on each side of breast and 2 leaves on each side of top of breast.
Rub turkey with remaining 4 tablespoons butter. Sprinkle with 1 teaspoon salt; season with pepper. Loosely stuff body cavity with 4 3/4 cups stuffing, and
neck cavity with 1/4 cup. Tie drumsticks together loosely with kitchen twine. Fold neck skin under body, and secure with toothpicks. Tuck wing tips under wings.
Roast turkey 30 minutes. Baste turkey; reduce oven temperature to 350. Continue to roast until an instant-read thermometer inserted into thickest part of the thigh (avoiding bone) registers 180, and stuffing registers 165, 2 to 3 hours. (If skin darkens too quickly during roasting, tent with foil.)
Let turkey rest 20 minutes before carving. Leave juices in roasting pan to make gravy.