The V8 diesel A8 and S-Class are pretty evenly matched.
Still, I think a V8 diesel is a little overkill, especially when the base S320 CDI is a pretty good deal. It's not slow and it gets even better fuel economy and is also fun to drive because of the 510 Nm of torque.
Yes, the S420 CDI will be brutal, but I like to keep it real[istic].
