This is an interesting topic for me now since I have just purchased a VE SV6 SIDI engine.
My first line of attack is thoughts of spraying a whole can of liqui moly petrol intake cleaner into the manifold & let it do what is does & have it resting on the intake valves in a hope that over numerous applications at each service, it would slowly clean them up. I use this as it's spray pattern goes everywhere instead of just using the nulon & subaru upper engine foam cleaners where it is only point & shoot & hope for the best..
Secondly, I guess we have to address the dirty crap being thrown back into the intake manifold, is this coming directly from the combustion chamber through the pcv or breathers?
I am playing around with a product called FTC Decarbonizer from Cost Effective Maintenance in QLD. It's a combustion catalyst that was designed to burn fuel faster & cleaner. It was made specifically for underground diesel engines to lower their emissions. So on petrols, it should work quickly.