CX901 is always full in Economy and always half empty in Business. All the Diamonds (and most of the Golds I think) get bumped up all the time. (I fly on it regularly). I wouldn't worry. You will be rebooked before you even land if the plane is late. And flights to Manila are often late themselves (not 901, but the afternoon flights certainly). HK is an easy airport to transit through provided you get moving as soon as you land.