I have taken the UA LAX/SFO-SYD 5 or 6 times. It has generally been early or on time in my recollection. The only time we were late was due to the fact that we had burned additional fuel and were arriving early - I believe that SYD does not allow traffic to come in before 6:30, so we were that early - and we had to stop in Brisbane to refuel as they had overconsumed (at least this is the story we were given). That little tour made us 30 mins late.

Couldn't you go LAX-SIN direct?