Cathay's engineering experience is not in doubt. Quite a few years ago, when Cathay started doing LGW-HKG as a non stop with 747-200s, Qantas sent Engineering staff to Hong Kong, because Qantas operated their planes at the same weights as CX, and QFstaff could not see how Cathay were operating the sector as a non stop. Does Cathay face stronger winds flying over the North Pacific than Qantas does flying from the South Pacific? Usually flying West is harder than flying East. Yes, we do know that Qantas will be doing MEL-LAX with 450 seats rather than the 520 seats that we thought they were going to use when the A380 was ordered. 450 seats is still a meaningful payload, provided the First and Business Class seats are occupied.
Cathay would have been disappointed with the A340-600 between HKG and NYC, though the A340-600HGW would have been better. It may be the the A380-800+ that will suit Cathay.