Heat broiler. Coat large broiler-pan rack with nonstick vegetable-oil cooking spray.
Mix brown sugar, allspice, black pepper, onion powder, garlic powder, thyme, salt and cayenne in bowl. Whisk in oil, molasses and lime juice until smooth. Add shrimp; toss to coat. Cover and refrigerate for 10 minutes, stirring after 5 minutes.
Arrange shrimp in single layer on broiler-pan rack. Broil about 4 inches from heat for 2 minutes. Turn shrimp over. Broil 1 to 2 minutes or until shrimp are cooked through.
Serve shrimp immediately with white rice if desired. Garnish with lime wedges. Pour drippings from broiler pan over shrimp, if desired.