To prepare pesto, put cilantro, garlic, lemon juice, Parmesan cheese, cayenne pepper, pistachios (minus 2 tablespoons), and salt in a food processor or blender. Add about half the olive oil and blend. Add remainder of olive oil and blend until mixture reaches desired consistency.
Bring a large pot of water to a boil. Cook pasta according to package directions; drain.
Return pasta to cooking pot on low heat, add pesto and chicken, heating until warmed through. Add tomatoes. Stir to combine.
Top with reserved 2 tablespoons of nuts then serve. Can be served warm, room temperature or cold as a summer salad.