Does the car have an Airleak or something else maybe that is causing the Turbo to overspeed? That seems very likely IMO. It should not be spanking turbo's at that rate, in any case.

Sorry if you already said and I missed, but in what way have the last 2 or 3 failed?

As Tony said, I run close to 2-Bar on what is considered one of the most delicate type of Turbo, the GT Series, and have never run a Dump Valve.

I used to run similar boost levels on my Stage 3 T3 too.

To be honest, yes to run a Dump Valve will increase your Turbo's chances of surviving, but when you stack all the other odds up against it, there are so many other things that can kill your turbo first.

Turbo warranties aren't worth sh1t in my opinion. As soon as a Turbo fails under warranty, it will have mysteriously been subjected to either a 'Foreign body' or 'Oil contamination'. Both of which are hard for you to fight anyway.

My money is on an Airleak. Start with your Throttle-body to Inlet Manifold Gasket.