As long as you get a good quality electronic gauge it should be accurate, and the tip should never snap off. (mine is around 5mm in diameter and looked perfect last time I removed it.) The material you tap it in to shouldn't make much difference, the thread is tapered so it will bite nice and tight into the tapped hole, as long as you use the correct tapered tap. If the Adaptronic has the ability to log 6 channels I would run a temp probe in each runner to allow individual cylinder trimming.
The max temps you want to see is dependant on timing. The less timing your engine is running the higher the exhaust temps will be compared to the combustion chamber. My engine runs very little timing under load as the boost is so high.
Imagine how soft the alloy pistons would be at that temperature, they would be so weak a little det would punch a hole straight through them. Luckily our pistons all run oil squirters underneath to keep temps under control, my stock pistons also have a donut shaped oil channel cast into them to help transfer the heat into the oil.