An A380 with a funny 6*** flight number arrived into YSSY from the North East the other night at FL420. Must have been empty to be that high!!!
Probably was fairly light on that occasion, however FL420 is easily achievable even with a reasonable load.
FL 420 can be comfortably and efficiently achieved at 380T and ISA+10, so with a ZFW of say 345T (400 pax or thereabouts) that would be doable 2 hours or so from destination if desired.