CX overbooks Y by a considerable margin on many routes, making op-ups Y-J and J-F far more common than on QF for example. Certain routes, days of week, and even time of day are 10x more likely to get opup than other CX flights, so it all depends. As for upgrade priority, that is a topic of wide debate on the CX forum, with some claiming that CX will opup all of MPC members first before doing Oneworld status opups. While I have never personally had an opup from CX as I only fly F with them, have seen many AA EXP opup'ed in LAX. SYD-HKG might have a lot of WPs flying, so SG may not be high enough status for opup on CX, even ignoring the CX vs QF status distinction. I'd expect ADL-HKG or even MEL-HKG to have much better opup chances than SYD-HKG.