I looked up the Qld Transport website:
http://www.transport.qld.gov.au/qt/faqs.ns...a25691000810ce5
Basically in order to get a slimline plate, first you have to "customise" your plate. This costs $65.80, and basically what happens is they make 2 new plates that are identical to the ones you already have, and you get to keep these for life, even if you change cars etc.
To get the slimline option, you pay an additional $33.90.
So, all up it will cost you $99.70 to get slimline plates that are pretty much identical to the ones you already have, except one of them is well... slimline.
Not worth it I recon...