I would expect the XR8 sedan to handle better than a Holden Wagon. Stock the LS1 needs to rev out a little before hitting it's stride.
there are many threads on how to improve the performance of the 5.7 (LS1) on varying budgets. If you are still interested in finding out more about the options for a SS Wagon there are many helpful people on this forum.
Unfortunately Ford doesn't have a V8 Falcon Wagon nor independent rear suspension. Still the 6 goes well and with a few mods could probably go allot harder. Probably slightly a little harder than stock 5.4 220Kw Falcon.
Subaru's are very well engineered cars, when it comes to space there is no comparison with the Commodore or Falcon.
If handling and acceleration is a criteria I'm not sure a large 4WD such as the Explorer would be any better than a Commodore or Commodore wagon. The BMW X5 Diesel has had many good reviews but that’s another price range.