Cook couscous according to manufacturer's directions. Thaw shrimp if frozen. Preheat oven to 350 degrees F. Tear off four 15-inch squares parchment paper. Spoon couscous on one side of each sheet of parchment paper. Layer the carrot, sweet pepper, green onions, and garlic next to the couscous. Top vegetables evenly with shrimp. Drizzle all with olive oil and balsamic vinegar and season with salt and pepper to taste. Add a sprig of thyme and lemon wedge, if desired, to each packet. Add a piece of butter to each.
Fold parchment over shrimp and vegetables; fold in the open sides of parchment several times to secure. Divide packets among 2 shallow baking pans.
Place pans on separate oven racks. Bake about 20 minutes or until shrimp turn opaque. Open carefully to check doneness, as steam will escape. To serve, transfer packets to dinner plates.