I was just reading something over at Corral Forums similar to this question. Most of the guys over there said 600rwhp is the magic number for a GT crank, some have ran higher, others no such luck...
There is no magic number on the cast crank, at least it has not been established yet. Everyone points to a number but I've yet to see anyone reference an actual crankshaft failure that wasn't the result of high rpms and a smaller balancer. If anyone knows of any please post the links. I'd love to read about them.