Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and simmer for 10-15 minutes or until tender.
Meanwhile, in a large skillet, cook beef over medium heat until no longer pink; drain. Stir in the tomatoes, salsa, green chilies and 1/4 teaspoon salt and pepper; heat through. Stir in 1/2 cup cheese. Transfer to a greased 4-cup baking dish; sprinkle with corn.
Drain potatoes and place in a large bowl. Mash potatoes with butter and remaining salt and pepper. Spread over top; sprinkle with remaining cheese.