I'd have a go at the o2 sensor then I reckon. A vacuum leak is more likely to make you lean rather than rich, as extra air is being sucked in. These cars have a pressure sensor rather than a MAF and are less likely to be impacted by getting dirty. (I think this also means they are less susceptible to leaks, as they are measuring overall intake system pressure including leaks, rather than air passing a particular point like a MAF where air entering/leaving after that point is a problem). Leaks before the turbo, eg airbox, shouldn't matter.