I did some quick experimentation:
Easter 2-7 Apr HKG-SYD Y(X): Standard - no, Priority 1 - yes, Priority 2 - yes
Easter 2-7 Apr HKG-SYD PE(T): Standard - no, Priority 1 - yes, Priority 2 - yes
Christmas 25-28 Dec HKG-CTS Y(X): Standard - no, Priority 1 - yes (except CX580), Priority 2 - yes (except CX580)
Christmas 25-28 Dec HKG-CTS PE(T): Standard - no, Priority 1 - yes (except CX582), Priority 2 - yes (except CX582)
I can't tell the difference between Priority 1 and Priority 2 right now.