My old post #39 over in the Sponsor/GB Classifieds section:
It's all about the combination of how the induction/ manifolding/ post turbo exhaust all works in unison. The small oe turbine sections (cast into the exhaust manifolds) are perhaps the Factory's way of keeping a lid on what they feel is a safe hp/tq limit.They knew going in that most tuners would come up on this stonewall of NOT being able to change out the hot housing with wrenches. After all they are in business to sell cars that stay on the road...and not the warranty bay.
But fear not...the winds of change are coming
Marty