Cook pasta according to package directions. Drain in a colander over a bowl; reserve 1 1/4 cups cooking liquid.
Heat a large skillet over medium-high heat. Add olive oil to pan; swirl to coat. Add prosciutto; sauté for 3 minutes or until browned and crisp. Remove prosciutto from pan with a slotted spoon; drain on a paper towel. Add red pepper to drippings in pan; cook 30 seconds, stirring constantly. Add reserved cooking liquid to pan; bring to a boil. Add 3 ounces goat cheese and salt, and cook for 2 minutes, stirring until smooth. Stir in grated lemon rind. Add pasta; toss gently to coat. Add watercress, and toss gently to combine.
Place about 1 1/2 cups pasta mixture on each of 4 plates. Top each serving evenly with remaining 1 ounce cheese, prosciutto, and black pepper.