There a quite a few tasty accessories and slight differences that the Holden's get, such as HIDs and some nice 10-spoke "luxury" looking wheels.

Im willing to be the wheels are a direct swap, but are things like headlights an easy switch? Any Aussie Holden owners on here?

Headlights are not interchangeable, because the Holden is right-hand drive and the headlights are designed to project differently. You'd be blinding oncoming drivers in left-hand drive USA.

rather have the GXP Rims, brembo brakes, LS3, and HID's

I can live with GXP rims and HID's
Anything else to me isnt needed on this great sedan

How about taillights? I love the GT engine but would rather the base taillights, or even better, the Holden Calais taillights...

Should be able to just re-aim the lights. But, you don't need to swap the capsules, should be able to just put the HID bulbs, ballast and wiring from the Holden models without affecting the aim.

Might be easier to get an aftermarket HID kit so you don't have to cut the stock wiring.

The towing packages available for the Commodores interest me, as one of them is rated at 2100kg (~4500lb).

And the HSV GTS front bumper cover with projector fog and driving lights. And bonus, it is still a split grille, though might have to work to get the arrow remounted.

The tails look to be the same, so shouldn't be an issue.

Brakes and suspension stuff from the HSV line would be nice, and would be a surprise if it did not swap. And I would imagine there are some out there interested in the various spoilers.

...so the big question remains then. When one finally gets his G8, where would one order thesre Holden parts?

a GTS spoiler would b a good idea on a G8

__________________
Current: 2006 Grand Prix GT S/C
Waiting for the G8 GXP

It's not a matter of aiming. The projectors are designed to throw more light to the left (to illuminate road signs). You can't re-aim them.

I've read elswhere the Calais tails are larger and can't be swapped for the G8 tails without also replacing the bumper cover.

It's a small difference, but the Calais lights do appear to be larger (look at the length from the crease below the PONTIAC G8 emblem):

Ah, I did not look close enough at the Calais tails. But the bumper cover should swap, as it is set up for dual exhaust. Not cheap though.

I would imagine there would be some SS-V owners looking with envy at the G8 hood. Maybe not many, but a few at least.