Lay tenderloins on cutting board with long side running parallel to counter edge. Cut horizontally down length of tenderloins, stopping 1/2-inch from edge so halves remain intact. Working with one at a time, open up tenderloins, place between 2 sheets of plastic wrap, and pound to 3/4-inch thickness.

Combine soy sauce, preserves, hoisin sauce, sherry, ginger, sesame oil, garlic, five-spice powder, and pepper in bowl.

Reserve 3/4 cup marinade.

Place pork in large zipper-lock bag and pour remaining marinade into bag with pork. Seal bag, turn to coat, and refrigerate for at least 30 minutes or up to 4 hours.

Combine reserved marinade, ketchup, and molasses in small saucepan. Cook over medium heat until syrupy and reduced to 3/4 cup, 3 to 5 minutes. Reserve 1/4 cup glaze for glazing cooked tenderloin.

Charcoal grill:

Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour evenly over grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.

Gas grill:

Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Turn all burners to medium-high.

Clean and oil cooking grate.

Pat pork dry with paper towels, then rub with vegetable oil. Grill pork (covered if using gas) until lightly charred on first side, about 2 minutes.

Flip and brush grilled side of pork evenly with 2 tablespoons glaze.

Continue grilling until lightly charred on second side, about 2 minutes.

Flip and brush evenly with 2 more tablespoons glaze. Repeat flipping and glazing twice more, until pork registers 140°F and is thickly glazed, about 4 minutes longer.

Transfer pork to cutting board and brush with reserved glaze. Tent loosely with aluminum foil and let rest for 5 minutes. Slice and serve.