Cook pasta according to package directions, omitting salt and fat. Add shrimp to pan during last 3 minutes of cook time. Drain and rinse with cold water. Drain.
Combine juice, sugar, Sriracha, and fish sauce in a large bowl, stirring until sugar dissolves. Add pasta mixture, lettuce, and next 4 ingredients (through cilantro) to juice mixture; toss to coat. Place about 2 cups pasta mixture on each of 4 plates. Sprinkle each serving with 4 teaspoons cashews.