Wrong. The VE Zeta is a completely new design that has not even been released. The sigma architecture is how old? 5years?
I think the delays are due to cost issues. The VE has an expensive 5 link rear suspension system. I suspect the delay is so that the platform can be reengineered to accept a simpler/cheaper rear suspension for us in Chevies (what sort of rear suspension does the Mustand have ? solid rear axil).
The VE based Zeta is expected to have an 8-10 year life. Holden was designing the VE before GM globalised. When GM globalised the VE it became the Zeta architecture and the US worked on designing US cars of the basic architecture.
On a side note it would not surprise me if GM starts building the GTO in the US. The monaro has offically stopped production however the US GTO is continuing on sale. I guess that Holden had to stop production of the Monaro to retool and re-organise the factory for the VE Zeta. So why not ship the tooling to the US and start GTO production there rather than just scrapping the tooling.