Speak to Brett at GCG in Sydney or Max at ATS in Melbourne - both hiflow the stock CT12A from bigger compressor/exhaust wheels to hybrid ported housings and replacement Garrett GT28 BB cores ; to what extent you go is up to you and your pocket.
There is also a new alternative just on the market - some brand new bolt on replacements for the CT12As with steel wheels and more boost capability.
I understand they are reasonably priced as well, however it's very early days as the first set was only recently fitted.
Have a word to Neil Griffiths from Rush Imports and keep an eye on the ALSC for more details as they are available.