In a Vitamix, or blender, puree the 2 cups of diced peaches and water. Mixture will be thick.
In a saucepan, add the syrup, whiskey, and peach puree. Put on medium heat, just till it begins to boil.
Turn heat to simmer and add the onion powder, garlic powder, salt and red pepper flakes (optional, see notes). Stir and simmer for about 3-4 minutes.
Whisk in cornstarch. Cover sauce pan and cook an additional 5 minutes.
Remove sauce from heat and allow to cool for about 15 minutes.
Marinate the chicken by placing in a heat proof container. Do not use a zip lock bag as the sauce will be hot. Place chicken flat in dish. Reserve about 1/2 cup of sauce and pour the remaining sauce over chicken. Cover and place in refrigerator for one hour.
Preheat and clean your grill.
Prepare the remaining 4 peaches (one per person). Make a small incision at the stem end of the peach, circling the circumference of the peach, cutting it in half. Pry the peach open and remove the pit with a spoon.
Brush the chicken and peaches with the reserved sauce and place the chicken, skin side down on the grill; and peaches, cut side down on the grill.
Grill the chicken for 10 minutes, then flip. Glaze again and continue to cook till reaching an internal temperature of 162º. Flipping as necessary and reapplying sauce.
Flip the peaches once grill marks appear. Grill peaches an additional 5 minutes. Remove.
Once chicken is at 162º, place on a platter and tent with foil, allowing the chicken to rest for about 10 minutes. It will reach 165º, a safe eating temperature.