the 4.2L A6 makes the right noise though, and has nicely inflated RS6 style arches
The A6 engine has some slight differences. Can't remember exactly what they were, but I know the ECU was different.
I believe they had a different inlet manifold, which was good in one sense as it got rid of the F-hose (as did the RS4 and very last 2.7T S4's) but I think it may have been a bit more restrictive.
Ask Musty over on AudiSRS. I think he did a lot of work on his A6 2.7T engine.