I wouldn't believe for a second that there will be "no V8" in 3+ years time. The media is only speculating.
Insignia would be no surprise. Holden are already bringing back the OPC variant (badged with the previously used Vauxhall "VXR" badging) as a low volume performance model, and as I've said on here, HSV are a decent chance of releasing a twin turbo Insignia in the future as their going forwards model.
Add the (still) stubbornly high aussie dollar that makes euro cars more competitive, and the dots are all lining up ready to be joined- starting with a super low volume, high performance "hero" model to get the market interested...