I tapped my EGT probe port into the cast iron turbo exhaust housing and it wasn't fun. It worked though but only just as the cast kept crumbling. I was too lazy to do it in the manifold and there was a perfect spot where the EGR would go if I had one. My Merc has its probe pre turbo and I feel this makes the most sense as it is the bore you are worried about rather than the turbo. Aluminium pistons expand faster than the steel bore taking the ring space and scoring the bore. As you say, lots of heat loss through the turbo so you are never really sure what combustion temps might be measuring post turbo. I guess if you lagged the exhaust and used a turbo blanket it might be closer?
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