Preheat a grill to medium high. One at a time, place the cutlets between 2 sheets of plastic wrap and pound with a meat mallet or heavy skillet until 1/4 inch thick. Transfer the cutlets to a bowl and toss with the cayenne, 1 teaspoon coriander, 1 teaspoon salt, 1/4 teaspoon black pepper and 1 tablespoon olive oil. Set aside.
Toss the plums with the remaining 1/4 teaspoon coriander, a pinch of salt and 1 tablespoon olive oil. Grill the plums, turning once, until just starting to soften, 3 to 4 minutes. Set aside.
Grill the turkey until cooked through, about 2 minutes per side.
Finely chop a few of the grilled plum wedges; put in a large bowl. Stir in the lemon juice, the remaining 3 tablespoons olive oil and 1/4 teaspoon each salt and black pepper. Add the arugula and toss. Top the turkey with the salad and the remaining grilled plums.