Yeah mate recently put a diesel gas aus system on my Gu 4.2 turbo. To be honest I wouldn spend 4 g on it to gain an extra 150-200 km. U do notice an increase in power when towing or on hill but a well set up custom turbo would be better in my opinion.
I installed a D-Gas system about 4 years ago and swear by it. My fuel economy increased about 30%, (more in some instances) and power and torque are up around 10%. On top of this, egt's are lower, the turbo spools up quicker and black smoke has noticably reduced.
At the time of installation, i was doing about 500km's a week so i ended up saving about $30 a week with the gas on. Considering the cost of around $4000 to supply and fit the complete system and the govt $2000 rebate i had it completely paid for in 18 months.....
The older style mechanically injected engines like my lump and the TD42's respond really well to a little gas so i say do it.