It would seem to suggest that the factory map doesn't manage the O2 sensors like a BMW factory map would, which sounds feasible.
A custom map may help, but engine mapping is a bit of a dark art. Best to find a place who specialise in engine mapping. There will probably be good stock and improved maps for the engine as I'm sure plenty of BMW folks map their v8's.
DMS in the uk offer a remap that will work on the 4.4 and 4.8 engines. They claim mor power too, but potentially at the expense of a little torque earlier in the curve based on the anecdotes of some owners..
