There isn't much!
I haven't seen many threads regarding carbon cleaning on the N55, and I can see why.
The valves are perfectly clean, and only a slight amount of buildup on the stems up top.
I might re-evaluate at 100K, but this is very clean for a direct injected motor at 70K. Some VW and Audi products are clogged up at 30K, so I'm very happy.