I'm basing the comment on several years of experience on the ORD-HKG route on the -400. Although we could usually make it non-stop after a gross weight takeoff, we could never carry a full payload. The max ZFW that we could manage was usually about 500K, although the aircraft limit would have permitted 535K. When we switched to the 777 on the route, we could always take enough fuel to make it non-stop, but the available payload was much less.

Joe