Ok so I dug up that old AMC (Issue No. 2) with an interview with Brocky about the car but there are no exact numbers as I had hoped.
Brocky himself apparently says "160 - 165 mph (down Conrod), which is about 265 - 270 km/h, or something like that".
He also says "I was driving for the Holden dealer, Bill Paterson, at the time and in the body shop at his dealership we actually built all the A9Xs for the factory... so I was involved in some of the body modifications and the fitting of the (wheel) flares and a whole lot of little things that had to be done to the cars when they arrived there."
He also says "Yeah, that (A9X diff) was out of the Holden One Tonner with Commodore discs adapted to it."