I was told by a guy from turbo techniques UK that Garrat / air research turbos are carbon sealed as standard. He seemed to know what he was on about, he reckoned he used to work as a mechanic for an F1 team. He said that he had never had any probs with the seals when drawthroughing thoses turbos.
I reckon a mitzi turbo would need carbon sealing, the way they burn oil I doubt weather the factory seals are even oil resistant. lol