The thing I've learned about IDIs is... More air + more fuel = more power. Also, for whatever power level you want, if you can add more air than needed, you'll get much better EGTs.
Also, boost doesn't blow head gaskets. It's *used* boost. So, 20 PSI on a stock 6.9 with stock fuel would probably be just fine(because none of that added air is actually assisting burning, so the cylinder pressures are much lower), but 10 PSI and fuel maxed would probably blow them in short order.
One other thing about head gaskets - Old gaskets are prone to leak. N/A or turbo, stress them and they'll fail. I'm thinking it's mostly just age really.
I've also taken a n/a 7.3 motor + factory turbo that had been rebuilt back in '01 or so, overheated it a bit, it started weeping coolant when hot from the head gaskets... Decided F*** it and installed my RD2-110 and just maxed the poor thing out(250 RWHP on the dyno, 320+ crank).
It lasted over a year before the gasket finally totally gave out. Motor was spotless inside when I took it apart.