I think 9 MM is a little short. I do not know what grade the original studs are. In my wagon, when I bought it the turbo was off but still with the car, nuts were missing. I kept the original studs but use stainless nuts. They hold up nice. If you only need to replace one I would stay with an 8.8 and use a stainless nut. I hardened stud would not stay hardened with the potential heat / temperature.

Did you rebuild your turbo because axial play or radial?
Did you measure your max boost pressure>

Mine has quite a bit of radial play and I would assume that the impeller is slowly grinding away.
I have haze of blue smoke and am wondering if the turbo is burning it .
Later J

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