A few years ago when I took my turbo off all but one of the studs snapped off. I ended up drilling all four of them out and using M10 (IIRC) Stainless nuts, bolts and heavy spring washers.

As I say, it's been like it for a few years and has been absolutely fine; they haven't come loose, snapped and do not show any signs of fatigue.

Sod paying ridiculous amounts for Inconel fasteners.