Cook the macaroni according to package directions until just al dente. Drain and set aside.
In a large saucepan over medium heat, melt the butter. Stir in the flour and cook for about 1 minute until smooth and lightly golden.
Slowly whisk in the milk and heavy cream, stirring constantly to prevent lumps. Cook until the mixture thickens, about 3–5 minutes.
Reduce heat to low and stir in 1 ¾ cups cheddar and ⅓ cup Monterey Jack cheese until melted and smooth.
Add the garlic powder, onion powder, salt, and pepper, then stir in the sour cream and mayonnaise until fully combined.
Fold in the drained pimentos and cooked macaroni, stirring until everything is well coated.
Pour into a greased baking dish and bake at 350°F for 20–25 minutes until hot and bubbly.
Sprinkle the top with the remaining cheese and return to the oven just until melted.
Let sit for a few minutes before serving.