Heat oven to 400°F; grease 2-quart dish. Cook macaroni in large pot with tablespoon salt to 1 minute short of package time; drain.
Meanwhile, warm pork, barbecue sauce, and water in covered skillet over medium-low 5 to 7 minutes until steaming, stirring against sticking.
Melt butter in saucepan; whisk flour 1 minute and gradually add milk. Add mustard, sauce salt, and pepper; simmer 3 minutes. Remove from heat; add cheddar.
Fold pasta into sauce; spread in dish. Loosely spoon hot pork over, leaving some cheese-coated pasta exposed.
Bake 15 to 20 minutes until bubbling and pork and pasta centers 165°F. Rest 5 minutes.