Lay tenderloins on cutting board with long side running parallel to counter edge. Cut horizontally down length of each tenderloin, stopping ½ inch from edge so tenderloin remains 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, sherry, ginger, sesame oil, garlic, five-spice powder, and pepper in bowl. Reserve ¾ cup of 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 ¾ cup 3 to 5 minutes. Reserve¼ cups glaze for glazing cooked pork.

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 paper towels, then rub with vegetable oil. Grill pork, covered, 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° and is thickly glazed, about 4 minutes longer. Transfer pork to cutting board and brush with reserved glaze. Tent loosely with foil and let rest for 5 minutes. Slice and serve.