Meanwhile, melt butter in a large skillet over medium heat. Gradually whisk in flour until smooth; cook, whisking constantly, 1 minute. Gradually whisk in 2 cups milk and next 3 ingredients; cook, whisking constantly, 3 to 5 minutes or until thickened. Stir in Cheddar cheese and pimiento until melted and smooth. Remove from heat, and stir in pasta.
Pour macaroni mixture into a lightly greased 13- x 9-inch baking dish. Stir together breadcrumbs and Parmesan cheese; sprinkle over mixture. Bake at 350° for 15 to 20 minutes or until golden and bubbly.