Cook macaroni according to directions till al dente and drain. Set aside. Preheat oven to 350. Spray a 9x13 pan with cooking spray.
In a large saucepan (4 quart) melt butter over med heat. Stir in flour, cayenne, ground pepper and mustard. Gradually stir in milk. Cook and stir mixture until it thickens and starts to bubble. Remove from heat and stir in 3 cups of the cheese. Stir until melted. Combine sauce with macaroni.
Pour into 9x13 pan. Sprinkle with remaining cheese and breadcrumbs. Bake 40 minutes or until hot and bubbly. Let Stand 10 minutes