The Cadillac 3.6 CTS is about $US35K in the US. When (if) it arrives here it will be about $US70K. So a $70K CTS-V - which can't be made in RHD anyway - would be probably $US150K if sold here. Holden could make the 427 RHD if they wanted to. Hell, HSV have LSA and LS9 engined mules running around and the next GTS will probably have the LSA too. Holden could probably build a 'G8' LS7 if they wanted.
FPV recently sold runout BF Cobra Falcons, only 400 sedans and 100 Utes if at $US65K each. Of a model that had died in the arse, no-one but died-in-the-wool Ford V8 buyers wanted; Ford's own turbo six at half the dollars is quicker. Buyers basically paid over the odds for a limited build car that was nothing but a stripe pack on a minutely more powerful GT Falcon.
They sold the lot pre-availability. They're already changing hands for more than they sold at full retail. HSV will sell 427 easily and within a year or so will be worth 20% more.
As far as Sigma vs Zeta? Holden developed both for GM and rejected Sigma as too expensive and narrow for a Holden at entry pricing. Zeta is really Sigma Mk II enlarged and made cost-effective. As Sigma is going, and future Zeta will feature alloy members which Holden has experience of, I guess you can see which one wins....