Got this reply from a tuner about egts from tuning. Opinions needed.

[mudski]

But don't the ZD30's not like extra boost.
Boom. For instance.

I can't talk for the crd's Mick but the Di's are fine with extra boost. Mine runs 25psi and has done for well over 12 months. The only issue with running higher boost is premature wear on the turbo. How much and how fast the wear is? Dunno.I have been told of the impellers busting off at over 25psi. I just checked my turbo as a pre trip check and it spins free and there is zero play in the shaft.
The "boom" you talk of I personally believe that's from under boosting. Hence adding the NADs to control this issue. Too low boost pressure and your egts sky rocket...I have now seen two motors that have dropped the pin, both had done the exact same damage. Melted number 3 and 4 pistons, plus other damage to the motor. The cylinder colour, you could tell it got bloody hot. And both were just cruising too with no stress on the motor. That's my personal belief on this theory.