Set roast in a rimmed dish and spoon soy mixture into slits. Pour remaining mixture over meat.
Set roast on a grill above a solid bed of medium-hot coals or on a gas barbecue set on medium-high heat (you can hold your hand at grill level only 3 to 4 seconds). Cover gas grill. Cook roast, turning once, until a thermometer inserted in center of thickest part registers 125° for rare, 20 to 25 minutes total for a 1 1/2- to 2-inch-thick piece.
Transfer meat to a board and let rest about 5 minutes. Cut across the grain in thin slices. Garnish with cilantro sprigs.