I was told specifically that only one of the variants of 777 operated by EK would be suitable due to performance issues. I'm presuming this is a 300ER version with the extended wingtips.
Clearly due to runway legnth then there are MTOW restrictions, though some of this restriction in payload can be offset by lower fuel loads due to distance NCL-DXB being less than half the 7930 NM range of a 300ER. Even with a generic model such as the 300ER then there are still variables due to power plant as as can be seen from published data from Boeing :-
http://www.boeing.com/commercial/air...s/7772sec3.pdf
I'd guess that it would only be the very occasional combination of temperature / wind direction & speed that would necessitate a review of pre-planned daily payloads leading to potentially embarrassing additional "non fuel" weight restrictions.