Preheat grill to medium.
Toss sweet potatoes with oil. Sprinkle with curry powder, cumin, 1/4 teaspoon salt and pepper and toss to combine.
Oil the grill rack. Turn off one burner. Grill the sweet potatoes on the hot side, flipping occasionally, until slightly charred, about 10 minutes. Move them to indirect heat and continue cooking, flipping occasionally, until tender, 8 to 10 minutes more. Transfer to a serving platter and sprinkle with the remaining 1/4 teaspoon salt.
Meanwhile, whisk coconut milk, peanut butter, lime juice, water, tamari (or soy sauce), chili-garlic sauce and maple syrup in a medium bowl. Serve the potatoes with the sauce and top with cilantro, if desired.
Nutrition Bonus: Vitamin A (186% daily value)