The reason thin walled china manifolds crack is due to expansion, they would crack just as easily if purged imo. If the manifold flange is split and the turbo/exhaust weight is carried by a well designed bracket of some sort, the manifolds would probably last for years (on road cars at least.)
I only purge exhaust pipes when the die grinder can't get to the back of the weld, this isn't food grade exhaust I am working on, and at $300 a bottle of argon I'm not going to waste it back purging unless I need to. I have never had any dump pipes or exhaust returned from weld failure either, track cars included.
One thing i can say, I definitely prefer to weld thin wall (1mm or thinner) while back purging as I am much less likely to blow holes and can crank much more current in. No doubt it is stronger, but this isn't a structural item we are talking about. Perhaps you get better pricing on Argon through your work Mick?