Preheat oven to 375 degrees F. Poke holes in each of the potatoes with a fork. Bake potatoes in the oven until fork tender, about 1 hour and 15 minutes. Remove from oven and set aside to cool. 

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In the meantime, cook the sausage in a skillet over medium heat. Break up sausage into bite sized pieces. Cook sausage until no longer pink. Using a slotted spoon, transfer to a paper towel-lined plate. Drain grease from skillet and return to heat. 

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Add butter to the skillet. Once butter has melted, add leeks, sage, thyme, and 1-1/2 teaspoons salt and cook until they start to turn golden brown and become tender, about 10 minutes. 

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Once potatoes are cool enough to handle, cut them in half and carefully scoop out the insides to within about 1/8-inch of the side. Place flesh in a large bowl and mash with a fork or potato masher. Add sausage, leeks, and sour cream. Stir until combined. 

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Spoon mixture back into the sweet potato shells. Top with Swiss cheese. 

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Arrange stuffed potatoes on a foil-lined baking sheet. Broil until cheese is melted, about 4 miuntes.