Place the sweet potatoes in a large pot of heavily salted water and bring to a boil over high heat. Reduce the heat to medium and simmer until a paring knife can easily be inserted, about 20 to 30 minutes. Drain the sweet potatoes in a colander in the sink and let cool slightly, about 10 minutes. Transfer to a baking sheet or large plate and arrange in a single layer. Refrigerate until cool, at least 1 hour. Meanwhile, heat the oil in a small frying pan over medium heat until shimmering. Remove the pan from heat, add the cumin and cayenne, and stir to combine. Set aside to cool. Combine the sour cream, lime zest, and lime juice in a small bowl. Taste and season with salt and pepper as needed; refrigerate until ready to use. When the sweet potatoes are ready, heat a grill pan or outdoor grill to medium high (about 375°F to 425°F). Meanwhile, halve the sweet potatoes lengthwise. Cut the halves lengthwise into 1/2- to 3/4-inch-thick wedges and place in a single layer on a baking sheet. Stir the oil mixture to reincorporate the spices. Brush the cut sides of the potato wedges with the oil mixture and season generously with salt and pepper. When the grill is hot, rub the grates with a towel dipped in vegetable oil. Place the sweet potato wedges on the grill in a single layer and cut-side down, cover the grill, and cook until grill marks appear, about 2 to 3 minutes. Flip the wedges to the other cut side, cover the grill, and cook until grill marks appear, about 2 to 3 minutes more. Transfer to a serving platter, dollop with the sour cream mixture, and sprinkle with the cilantro. Serve immediately.