Believe I'm right in saying the -400ER's are used on routes into SFO aswell. Definately saw one parked up on day-stop there a few months ago.
In terms of the runways used at LAX, assuming it's physically possible to get the A380 to the southerly runways, why would you not use the extra 600m of 25R for such a heavyweight departure? Surely a few mins extra taxiing is going to make little difference to block time on such a long flight?