Personally I prefer a bit more time, just to stretch my legs and maybe use a non-bouncing toilet, but you should be fine.

There's always the chance that your flight to HKG will arrive later than scheduled (for whatever reasons). It's never happened to me but I assume CX will get you to SIN somehow; either on a different carrier or on the next available CX flight. If it results in a downgrade from F you should be able to get some sort of compensation.

EDIT: Not sure if you booked this as a single ticket or two separate ones, that might make a difference. I'm assuming that you don't have to clear customs/immigration in HKG and, also, that your bags (if any) are checked all the way through to SIN.