Someone more knowledgeable will be along soon, I'll chip in in the meantime
Petrol D2s have the good old injector in manifold system, meaning the injector nicely sprays the tops of the valves all the time. Make sure you do WOT now and then so the injectors do a full blast of fuel over the valves.
For this reason I really doubt D2s have much carbon deposits.
Certainly I doubt they have the troublesome deposits on top of the valve which some new FSI engines suffer from. This is the problem type deposit because it can cause problems with valve seating and reduce air intake volume?
FSI injectors are inside the cylinder so never spray the tops of the valves.
So my guess is, total waste of time and money on a D2
Unless it was run on cheap/bad fuel for a long time, and used non-synthetic oil.