What's your oil consumption like?
Mine used to chuck out a lot of soot and I could see in the mirror on hard acceleration a plume of stour/dust coming out the exhaust. Tail pipe and bumper sooty.
Mine had high oil consumption, changing to thicker oil helped but still high, double the recommended at 2 litres per 1k miles - eventually pinned this down to blown oil seal in the hot side of the turbo.
Now I barely have any soot on the exhaust & oil consumption has quartered.