Whisk 1 ¼ cups mayonnaise, Dijon mustard, ketchup, Worcestershire sauce, salt, and cayenne pepper together in a bowl; add lemon juice and 1/3 cup chopped dill. Whisk until thoroughly combined; keep refrigerated.
Bring a pot of well-salted water to a boil and stir in pasta shells; cook until tender, 8 to 10 minutes. Drain and rinse with cold water to cool pasta slightly; drain again. Transfer to a large bowl.
Toss shrimp with pasta; add red bell pepper, celery, and dressing to pasta mixture. Mix thoroughly to coat and fill shells with dressing. Cover bowl with plastic wrap and refrigerate until chilled, 2 to 3 hours.
Stir salad again before serving and season to taste with more salt, black pepper, lemon juice, and cayenne pepper if desired. If salad seems a little dry, mix in a little more mayonnaise. Garnish with paprika and sprigs of dill.
