Combine first 7 ingredients in a bowl; stir in 1/4 teaspoon salt and 1/4 teaspoon black pepper. Let stand 30 minutes.
Lightly coat both sides of tuna steaks with cooking spray; sprinkle with remaining salt and pepper. Place steaks on grill rack; grill 3 minutes on each side or until fish is medium-rare or desired degree of doneness. Serve with salsa, and, if desired, lime wedges.