it had slipped my mind you had a passat. so its the same engine as the jetta, 1z, or ahu. the tune is the same, engine management. is the gearing the same? as the mk3 jetta? just to ask w/o doing research, perhaps the trans case has some differences. other than that its your m-TDI conversion, and turbo, i forget bigger turbo? along with some mods, and your tune.
getting some info together on this subject. for one my mk3 jetta, as is well known, has fuel mileage from 40-42.x mpg, in which case i have no idea why the passat has better mileage(as stock), and your m-TDI is killing it. in the future i have some options for a conversion(s).