Preheat oven to 350 degrees F; use a fork to poke some holes in each sweet potato, then wrap in aluminum foil.
Bake until tender, about 45 to 60 minutes; unwrap, slit down the middle of each potato and let sit until cool enough to handle.
Meanwhile, increase heat to 450 degrees F; place grapes on a baking sheet, drizzle with oil, season with a pinch each of salt and pepper.
Roast until they begin to burst, 20 to 25 minutes; remove and let cool.
By now the sweet potatoes should be cool enough to handle, so use a spoon to gently remove the flesh while leaving the skins intact; mash potatoes with 3 ounces of goat cheese, cinnamon, nutmeg, salt, pepper, and honey.
Taste for seasoning, then scoop flesh back into skins (re-heat in oven if desired), top with remaining goat cheese, add grapes, and drizzle with honey.