Place tenderloin in a large zip top bag. Combine soy sauce and next 6 ingredients; stir well. Pour over the tenderloin. Seal bag securely. Marinate in the refrigerator for 8 hours, turning occasionally.
Remove tenderloin from the marinade, discarding marinade. Grill, covered, over medium hot coals (350 to 400 degrees) for 20 to 30 minutes, or until the meat thermometer inserted into the thickest part of the tenderloin registers 160 degrees, turning occasionally.
Combine 1/4 cup brown sugar and the next 4 ingredients in a small saucepan. Stir well. Bring to a boil.
Combine the cornstarch and water, stirring until smooth. Stir the cornstarch mixture into the brown sugar mixture, and cook, stirring constantly, until thickened.
Slice the tenderloin, and place on a serving platter. Sprinkle with the peanuts, and serve with the warm sauce.