Prepare a medium-hot grill fire.
In a medium bowl, combine the tomatoes, cilantro, scallions, lime juice, 1 Tbs. of the oil, 1-1/2 tsp. of the sugar, and the lime zest. If your tomatoes aren’t perfectly ripe and sweet, add the remaining 1/2 tsp. sugar. Season with 1/2 tsp. kosher salt and 1/4 tsp. pepper, or to taste.
In another medium bowl, mix the chipotle, the remaining 2 Tbs. oil, 1/2 tsp. kosher salt, and 1/4 tsp. pepper.
Trim the chicken. If the tenderloins are still attached, remove them and save for another use. Use the flat side of a meat mallet to pound each chicken breast to an even 1/2-inch thickness. Add the chicken to the chipotle mixture and toss well to coat.
When the grill is ready, lay the chicken on the hot grill grates and cook, covered, until the chicken has grill marks and the edges turn opaque, 2 to 3 minutes. Flip the breasts and continue to cook until the chicken is cooked through, 2 to 3 minutes more.
Transfer the chicken to a clean cutting board and let rest for 5 minutes. Slice each breast crosswise on the diagonal into 1/2-inch-thick slices. Arrange the chicken on a platter and top with the salsa.
If you're serving at room temperature, the chicken can be grilled ahead. After grilling, let it cool for 20 minutes, refrigerate (for up to 8 hours), and slice just before serving with the salsa. The salsa ingredients may be prepared up to 2 hours ahead, but mix them together just before serving.