Preheat the broiler. Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Drain, then transfer to an 8-inch square flameproof baking dish.
Meanwhile, cook the sausage in a medium saucepan over medium-high heat, breaking up the meat, until browned and crisp, about 10 minutes. Add the flour, chili powder, cumin and 1/2 teaspoon salt and cook, stirring, until combined, about 1 minute. Slowly whisk in the milk, scraping up any browned bits. Add the processed cheese and whisk until melted and smooth, 1 to 2 minutes. Remove from the heat and whisk in the eggs. Return the saucepan to medium-low heat and cook, whisking, until thickened, 1 to 2 minutes. Whisk in the sour cream and tomatoes.
Pour the cheese sauce over the pasta and stir to combine. Sprinkle with the shredded cheese and broil until melted and bubbling, 2 to 3 minutes.