Preheat oven or grill to 425°F.
Microwave halved potatoes for 5–6 minutes or boil in salted water for 8–10 minutes. Let cool.
In a large bowl, toss together shrimp, sausage, corn, onion, garlic, cooled potatoes, seasoning, garlic powder, oil, salt, and pepper until well coated.
Tear off 4 large sheets of foil. Evenly divide the mixture among them.
Top each packet with a lemon wedge and 1 tablespoon of butter.
Fold foil over contents, sealing all edges tightly to create leak-proof packets.
Grill or bake for 15–20 minutes, until shrimp is pink and cooked through.
Carefully open packets (watch for steam!), sprinkle with parsley, and serve with extra lemon if desired.