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I think he's saying that he's been informed that the Commodore 1600kg bar differs from the Calais 1600kg bar - and is asking whether that's true (and if so - how).
That would be my guess as well. Maybe the receiver is a little longer or shorter due to bumper profile. Another possibility is if the reference is to a base Commodore 3.0 where the car is rated no more than 1600Kg, whereas a 3.6 as in a Calais and SV6 are 2100Kg if weight Distribution Hitch is being used. In the case of factory bars the only difference for 1600Kg or 2100Kg is the addition of Weight Distribution Hitch.
I'd say not that difficult, but you do need to remove the bumper to fit the tow bar. I've done it on a VE, on the VE there is a phillips screw right at the top of the bumper in the wheel arch, and also I think two of those push pins (scrivets I think they are called) inside the rear wheel arch. There is also 3 scrivets at the rear bottom of the bumper. When you remove these, to get the bumper off, from either side at the top of the rear wheel arch give a firm pull of the bumper away from the body and repeat on the other side - this will leave the the bumper still attached across the rear, similar to the sides pull the bumper away from the body - there are clips a little below the boot lid, like the sides, the bumper needs to be pulled away from body to release the clips. Once the bumper is off you'll see an aluminium bar across the back of the car below the boot lid. That aluminium bar needs to be refitted to the tow bar once it's been installed. There are bolts under the boot carpet to allow that aluminium bars brackets to be removed. If it's just a 1200Kg bar you don't need to remove the rear mufflers (or for a VF Omega I assume just one muffler), if it's 1600 KG bar then there is a bolt to be fitted directly above the muffler, so the muffler needs to be removed to install that bolt on each side.