Sprinkle tuna with 1/4 teaspoon salt and 1/4 teaspoon pepper. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place tuna on rack; grill, covered, about 5 minutes on each side or until fish flakes easily when tested with a fork. Remove from grill; cut tuna into bite-size pieces. Let cool 10 minutes.
Combine remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, mango, and next 5 ingredients. Add tuna; toss gently. Cover and refrigerate 30 minutes or until thoroughly chilled.
Place a lettuce leaf on each of 4 individual serving plates. Spoon about 1 cup tuna mixture onto each lettuce leaf.