Combine first 7 ingredients in processor. Add 4 tablespoons mint; blend until smooth. Transfer to large bowl. Gradually whisk in 3 cups broth. Season soup to taste with salt and pepper. Cover; chill until cold, about 2 hours. (Can be made 6 hours ahead. Keep chilled. Thin with more broth by 1/4 cupfuls, if desired. Rewhisk before serving.)
Ladle soup into bowls. Garnish with diced tomatoes and remaining 2 tablespoons mint.