18"s are optional, as are side airbags I think (check the
www.ford.com.au website.)
As the VY SS is essentially indentical to your VX SS, albeit with 10 more kW and a facelift (which you may or may not like) then I'd compare your current car after test driving a an XR6T/XR8 and see what you think of them on the road.
If you're into modifying your cars, then all 3 engines have lots of scope for improvement but it seems the XR6T has the best bang for buck in that department.