I'm unsure if a cracked turbo shaft could do this, and have never heard of that particular defect.
Replacing the studs which hold the exhaust manifold to the head will have several benefits:
Engine noise will be reduced.
Smelly exhaust will no longer enter the passenger compartment.
The engine will feel more responsive due both to having no exhaust leak and getting the full boost available.
If helicoils are too expensive, perhaps you can buy some oversize studs (I don't know what size, sorry)and drill and tap the holes for new, bigger studs.
I have not done this, but this may be a good possible fix.
Best of luck.