Sounds like the VDO's are the right option
I have driven a HSV with the T56 and found it incredibly slow and heavy, it's actually a common complaint amongst HSV owners. I am not expecting the change to be as nice as my gtr but I was kinda expecting something a little less "puts flame suit on" agricultural.
While I haven't driven a XR6t or an XR8 with the Ford revised T56 all reports say that it has a much nicer shift, although not 100% sure about ratios with the gtr diff.
I spoke with someone at holinger and they say the Ford box is better due to double syncros and revised gear face, so if the Getrag box falls into the too hard basket I will be chasing the ford T56.
Also of note is the T56 Mal Wood produces for Cobras with the shift moved to a center position to get around the normal T56 problem of requiring double jointed elbows
Std T56 shift position
Cobra Modified center shift
Jamie