The STAR arrivals are being progressively changed to allow for accommodation of the Airways PBN change over. The public RNP AR approaches should be up and running at AKL this year as well so that eventually the instrument approach will mirror the visual approach.
Some tests were done on RTA arrivals at least 6 years ago and due to the shorter sectors on WLG AKL for the jets there was just not the flight time available to make any real difference without huge drop down in altitudes to reduce speeds, and RTA holding on the ground at WLG was not an option.
AH - We are told that Enroute ATC is required to deliver jets to AKL arrivals (126.0) at a minimum 10 mile separation. The difference in speed between an A320 (.78 you say) at 2000 feet above a cruise B737CL (.75) is not great enough time available to catch up and get passed with 10 mile separation at approximately 120 AA. What I see a lot of is the approach controller uses this gap between jets to put a Beech in and so really screw up the second jet!
However I do agree with you the "first ready and able to use the runway" is not a good departure planning method. How many times does CHC TWR (also know as Mount Cook TWR) fire off an ATR in front of a jet only to have the Jet RADAR vectored all over the sky to get through the ATR altitude.
It would be a simple matter to taxi the slower B733 to A7 or A2 and fire off the A320 from A6 or A3 and save lots of stress all around!
The real issue is that AKL has too many different performance types all trying to use the same airspace and ATC always protect the missed approach of Jet behind TP hence the big spacing.