Place frozen popcorn shrimp in a large resealable plastic bag. In a small bowl combine melted butter, garlic powder, and Cajun seasoning. Pour seasoned butter over shrimp. Seal bag and gently shake to coat shrimp.
Place popcorn shrimp onto a large baking sheet and bake according to package instructions.
Meanwhile, in a small bowl combine mayonnaise, hot sauce, pickle relish, and mustard.
When shrimp is cooked, place a slather of the Cajun mayonnaise sauce onto the base of a hoagie bun (I like to toast mine with a little butter first). Add shrimp. Garnish with lettuce and tomatoes.