Well I definately get smoke coming out the back on hard acceleration - more noticeable at night when you look in the rear view mirror and see it through the car behinds headlights. I don't know if it's thick and black or just greyish. I did see a passat TDI next to me the other day that booted out some thick black crud.
If I had another horrible problem just as a head gasket, surely I would see, hear and smell this on tickover when I put my head down next to the exhaust??
I just think with the hissing and poor MPG it's got to be a boost leak like Rhyso says.