The smoke you see coming up out of the hood on the passenger side cowl grill area is from the valve covers leaking. The oil drips out (especially if you are on an incline) and runs down the side of the motor and drips right onto the turbo housing. That is what you see on the outside of the turbo and has nothing to do with the internal seals. I just did this job on my Z - it's not the easiest, but not that big of a deal. I lived with the leaking and the smoke until I needed to do an injector change - the time to fix it is the next time the plenum is off. You'll need new intake valve cover gaskets, new rubber half-moon plugs (4) and liquid gasket for the exhaust valve covers.
Turbo seals are internal to the turbo and will leak into the intake or exhaust tract. The oil gets around the turbine shaft seal and drips. I expect that a small amount of this happens anyway and should not cause blue exhaust smoke or a noticable decline in the engine oil level between changes. I don't know how quickly this becomes a big problem; I saw less than 1/4 teaspoon of oil pooled in the bottom of the compressor housing when I last had my charge pipe off and a little bit of residue on the inside of the pipe and I'm not overly concerned as the car has 120k miles on it. I would not replace them for that. If they boost fine and given the mileage of the car, I wouldn't think there is much danger of them coming apart.
