1. Preheat oven to 350°F. Coat a 13- x 9-inch baking dish with cooking spray; place on a baking sheet lined with aluminum foil, and set aside. Fill a large pot with salted water, and bring to a boil over high. Add pasta, and cook according to package directions. Drain.
2. While pasta cooks, melt 6 tablespoons of the butter in a large saucepan over medium. Add onion; cook, stirring often, until just tender, about 5 minutes. Stir in flour; cook, stirring constantly, until onion is coated in flour, about 1 minute. Gradually whisk in milk, mustard, garlic powder, and black pepper; cook, stirring constantly, until mixture thickens slightly, about 10 minutes. Remove from heat. Gradually stir in Cheddar and Colby-Jack cheeses in ¾-cup increments until melted and smooth, about 2 minutes.
3. Stir pork and drained pasta into cheese mixture in saucepan. Spoon into prepared baking dish. Microwave remaining 2 tablespoons butter in a small microwavable bowl on HIGH until melted, about 30 seconds. Stir in panko until combined. Sprinkle evenly over pasta mixture in baking dish.
4. Bake in preheated oven until browned and bubbly, about 30 minutes. Remove from oven; let stand 10 minutes. Sprinkle with scallions. Spoon evenly onto serving plates; drizzle with barbecue sauce, and serve.