You can see above there are example F menus from LAX-HKG posted - it is a good selection - the "worth pre-ordering" concept is only going to be a good idea if you are very keen to eat lobster, as otherwise you are going to be turning down the option of choice on-board. As far as I know Cathay caters F properly (at least in my experience) and I haven't had them run out of meals, so you can always have your first choice. So be pre-ordering you guarantee yourself a lobster (appeals to some) at the cost of spontaneity, multiple choices and seeing how you feel on the day, hopefully after you've already stuffed yourself at the QF LAX lounge, which has a great food selection freshly cooked for you and will definitely have a few fish and shellfish options - but likely not lobster..