I stand corrected about the distance though DXB to LAX is a polar route and at this time, EK doesn't have approval for flights above a certain latitude (not sure exactly what latitude or if they intend to seek approval). I do stand by my statement that the time zone change is going to be the real killer.
As far as airplanes being payload limited or not - it is a bit of a simplistic statement since fuel taken is also a function of required alternates, some of which might be quite a distance away. If we could always flight plan with EWR as an alternate, I'm sure that payload generally wouldn't be a problem. My understanding of the difference between the 777-300ER/200LR and the A340-500 is not necessarily how much payload they can lift, but what they can lift profitably. I may worry about a lot of things but I don't worry about our commercial department getting their numbers right. If the A340-500 could do DXB-LAX more profitably over the course of a year than the B777-200LR, then I'm sure it would be flying it.