Heat oven to 400°F. Line cookie sheet with foil; spray foil with cooking spray. Pierce sweet potatoes with fork; rub with oil. Place on cookie sheet. Bake about 45 to 55 minutes or until tender when pierced with fork.
Meanwhile, in 12-inch nonstick skillet, cook bacon over medium-high heat until crisp; remove with slotted spoon. Cook onion and celery in bacon drippings over medium-high heat for 1 to 2 minutes, stirring frequently, until softened. Add broth and 2 tablespoons of the butter; heat to boiling. Stir in stuffing mix, bacon, cranberries and 2 tablespoons of the walnuts. Remove from heat; cover until needed.
Remove sweet potatoes from oven. Reduce oven temperature to 375°F. When potatoes are cool enough to handle, cut in half lengthwise. Using a melon baller, remove potato flesh, leaving 1/3-inch-thick wall of potato on inside of shell. Set potato shells aside. Place potato flesh in large bowl. Add 2 tablespoons syrup, remaining 2 tablespoons butter and the nutmeg; mash. Spoon about 1/2 cup stuffing mixture into each potato shell; spoon mashed sweet potato mixture over stuffing, leaving some stuffing exposed around side of shell.
Line cookie sheet again with foil; spray foil with cooking spray. Place potato boats on cookie sheet. Bake 15 minutes or until hot. Sprinkle potatoes with remaining 2 tablespoons walnuts; drizzle with additional syrup.