Most rear brakes are the same in the sense that they are all single piston. Until you get to the second gen hsv's and they started adding the 4 piston front and rear brakes. As for v8/hsv brakes I think they might have slightly bigger rotors at the rear. Vt onwards has 286mm rear rotors where as vn - vs have about 278mm.
Most people don't really upgrade their rear brakes and just upgrade the fronts. There's not a whole lot to gain. A front brake upgrade and slotted rotors all round with some decent pads will really increase your stopping power.
Not 100% sure if live axle brakes would bolt on to IRS. Wouldn't think so though?