The 4.2 (and 2.7 I think) have had the aluminum parts since introduction in the US. My early 2000 4.2 certainly does. Per a prior post the intro date was different (later) in Europe and elsewhere from what I've read in the Audi worldwide site press release archives. The 2.8 (now 3.0) was cut over to these later than the 2000 introduction of the 4.2 and 2.7 in the US. The aluminum stuff was first rolled out in the A8 a year or so earlier in response to a lot of handling cricism from what I saw in the press.
S6'ers will have to comment on the springs and shocks in the field. The Audi press releases have cited a 30% increase as I recall, but they've also said that in sports suspension related
PR--you would have to check it at the parts # level perhaps--harder to do here apples to apples where the sedan is a 4.2 and the wagon the S6.