in fact it was one of their main concerns when they designed these manifolds... they had Greddy ones that cracked within a month. They might eventually crack, but I expect they'll last a real long time given all these Zs will hardly be daily drivers...
I always said the Pentroof ones were a steal, and much harder to make than they look, its just hard to persuade people when they've never seen one in person :)
Can't wait to see your dyno too, you should have more top end power on turbos alone given the larger exhaust housing :) Plus there is some more power to be had in the colder weather, this Pentroof dyno was at 70F.