I think coming back the other way might be the problem though.
Headwinds at the lower latitudes would be a lot stronger than LAX-SYD which spends the majority of the flight in the relatively light winds of the tropics.
SYD-JNB at 5963nm is probably a better comparison. The standard -400s fly that route but it can be a bit tight, particularly if there's TEMPO TS on Joburg.