From the info available on the boeing website,
767-400ER 296 pax 2 class OEW 103872 (351 kg/pax)
787-8 242 pax mixed 3 class OEW 117798 (487 kg/pax)
even if you would fit the same number of pax (296) in a 787-8 its still heavier by about 50 kg per seat.