Each of the Audi/VW FSI motors will have build-up issues. It doesn't matter if its the 2.0, 3.6, or 4.2... they will all be victim to this issue.
As far as dealer repair... it would be fair to say no one really gets it done at the dealer. Most people, if not doing it themselves, will find a local independent shop to do the cleaning. There really isn't much to repair, unless you have other broken parts that may be contributing.
If you search various forums, you will find quite a few DIY posts on how to remove the manifold, and what tools/chemicals are useful, so that you can clean the backs of the valves yourself.
On my 4.2, I found that the removing of the manifold was relatively easy, when I followed the DIY I found on another forum. The cleaning process is long, but not difficulty. Just a lot of spraying, soaking, scraping and then repeating... over and over again.