Cook pasta according to package directions; drain and set aside.
Melt butter in a large pot over medium heat. Whisk in flour, salt, and pepper; cook for 1 minute stirring constantly.
Slowly whisk in milk until smooth, stir continuously until liquid thickens and comes to a low boil. Add garlic & onion powders. Reduce heat to low and stir in 4 cups of cheese.
Add cooked pasta to cheese sauce.
Heat chicken and BBQ sauce in the pasta pan until just heated.
Pour mixture into 13 x 9 or 2 smaller baking dishes then top evenly with BBQ chicken.
Sprinkle with 1 cup of cheese and bake for 25-30 minutes at 350 degrees.