This topic has been beaten to death and beyond on the c63 forum in this thread, which may provide some useful information:
http://mbworld.org/forums/c63-amg-w2...manifolds.html
As for the m24 and rd-sport headers, I don't think its accurate to speculate about the performance gains on an N/A application and compare those to an F/I application. In N/A cars, a larger diameter primary could produce negligible gains, or even power loss because with the increased diameter, because there is also a decrease in exhaust velocity once the piping diameter is too large. Bigger is not always better. In F/I, the opposite case may apply. I have seen an e9x m3 with F/I and custom-made headers installed, and the owner did report gains, however I never saw a dyno sheet to confirm. I think there would be gains at higher boost levels, however the gains on N/A, even if they were there, probably would not be worth the expense.
On the c63's m156, there are big gains because it comes stock with log-style manifolds. On the s62's, which already comes with equal- length headers, the only place for improvement probably would be increasing the diameter of the primaries or a set of long-tubes.