Given the A350-900's empty range and the loading charts I've seen, given LAX-SYD is about 6,500nm and take off from basically sea level, with reserves (call it 7,000nm range), payload is about 40 tons, maybe a ton or so more.

Using the standard 95kg/pax measure, DL's 306 pax configured A350-900 load would be 29 tons. Even being a little conservative on the 95kg figure and adding a little cargo, there's plenty to spare. Full pax plus a bit of cargo is easily do-able unless the winds are highly unfavourable.

DL would not be carrying max structural payload unless it was carrying dense cargo. Therefore I'm not sure saying that the 350 would be "severely load limited" is entirely correct.

Sure the B777-200LR is capable of more lift but with a fair bit higher trip cost.