Thaw shrimp, if frozen, then peel and devein shrimp. In a medium bowl combine lemon juice, chili powder, cumin and pepper. Add shrimp and toss to coat. Let shrimp stand for 20 minutes, stirring occassionaly.
Meanwhile, bring water to boil in a large saucepan. Break noodles into bite-sized pieces. Stir 1 seasoning packet and the broken noodles in the water. Discard remaining seasoning packet. Return to boiling and cook for 1 minute.
Add the shrimp and cook 1 to 2 minutes longer, or until shrimp turns pink. Stir in salsa, beans and corn and heat through.
To serve, top each serving with cilantro if desired.