Steve,
Yep, that's what the big man has seen on more than one occasion (but I don't know how many times). Some of the GT range that are smaller than the T4 also MUST run them, as the shaft diameter is tiny (so no strength). Apparantly it is all down to this dimension and the amount of airflow the turbo can produce at peak boost. The T4s have sufficient flow and small shaft size for the stall effect to shear the shaft in high boost applications, hence the d/v requirement. The TO3/4Bs don't have the flow capability to do this kind of damage.