I'd definitely try one of the additive-cleaners which specifically claim to remove combustion chamber deposits to some extent. This seems well covered in the previous posts.
Another friend in addressing this matter is your gas pedal. In general, Euro engines like your Passat (although I don't have first-hand experience with that engine) tend to benefit from aggressive driving on occasion. Some of these engines, designed primarily for Autobahn type driving and revved with manual trannies, tend to carbon up when driven gently here in the States. A few years ago my friend, a former Trans-Am racer, bought an Alfa Romeo Milano V-6 in Alabama. Owned by an older woman who drove slowly, the car was a bit lethargic. He dumped some additives of unknown type in the tank, and then practically full-throttled the thing back to the West Coast. By his arrival, the car had definitely gotten spunkier, so he definitely blew a periodical chart's worth of junk out the tailpipe.
My suggestion would be to first add some cleaning agents to the tank, and then essetially drag race the thing to redline for a week or so wherever you go. Thereafter, simply do the same on occasional freeway on-ramps.