Comparable VW petrol engines, in terms of power and very close torque, produce less co2, mostly because a petrol engine can be lower displacement.
What? Who told you that? That is literally the opposite of the truth. For about the same level of driving experience (less power, more torque, make it less immediate but more relentless) you have a much smaller engine. OR, and this is a bit hilarious, you build a gasoline engine with all the drawbacks of a diesel (AKA direct injected gasoline) but without the benefit of being able to run on diesel fuel.
Once you no longer ignore the carbon in the diesel soot, diesel stops looking good,
False, ignoramus. In fact, once you no longer ignore the almost-entirely-PM2.5-and-thus-most-carcinogenic gasoline soot, gasoline stops looking good. When you consider the additional unburned HC released during a typical gasoline drive cycle (not just at startup, but also at WOT) which is the most harmful automotive emission, gasoline starts to look positively pathetic.
and the higher power/displacement of petrol shows advantages at part load.
It does no such thing. Look at any vehicle offered as both a gasser and a turbo diesel. They always have a smaller engine as a diesel, literally always. You have this completely backwards. And Subaru's diesel engine isn't even heavy, so they don't even have to be that!