QF are short of ULH capable aircraft. All long hauls are balancing loads with departure altitude ( remember JNB is 1750 m ASL ). Apparently the 380 is more flexible than the twins in this regard. Both the 789 and 380 can easily do the range but I don’t know whether the 789 has proportionally less capacity for a full pax/ freight load than the 380. QF insiders would know more.