I don't think he's complaining about the speed of the shift. More so the time it takes for the engine to rev out to the shift point. Only 3 things you can do here buddy if you want to keep your car an auto.

1) 3.45:1 or shorter diff gears (4.11:1 on a stock or near stock street car is just stupid cheap s**t tight a*** boy ricer if you ask me. 3.5 camera phones of awesomeness)
2) Small tires. Goodyear grand rally s trailer tires 165's on 14" rims would help your cause (if you thought 4.11:1 diff gears on a street car were a tight a*** boy racer mod then trailer tires on 13-14" rims is the epitome of dept of housing performance. No amount of camera phones cane describe the awesomeness)
3) More power in the form of an aftermarket cam, ported head, exhaust..... all the good stuff that allows your engine to breather better are what's needed. Even forced induction if your situation and budget allows it (0 camera phones of awesomeness).

So there you have it. Some engine mods and a set of 3.45:1 diff gears would be the best bet to help your cause.

Oh and one other thing I forgot to mention. If you really love the short 1st gear so much on the commodores and you have more dollars than cents. Look into fitting one of those boxes behind a falcon.

Is a Zf 6 speed possible?

what everyone else has said. Shiftkit will help it change alot quicker. I have 3.9's in mine and it not too bad on the freeway. Sits on 2300rpm at 100k's and gets around the same economy if not better than when i had 3.08 diff gears.
If i were you i would go 3.7 or 3.9 diff gears, adjustable shiftkit and a trans service. Just to make sure everything is working as it should.