Geez. After watching the market for close to two years and giving up, i've found that that these only sell if they are:
Sedan NA: 15k or less
Coupe NA: 20k- 25k
Gtt Coupe: 40-50k
Gtt Sedan ~30k
Any GTT above 50k has never been sold with all I have seen on carsales, autotrader, gumtree, private sellers etc.
I've also seen a tuned GTT stated to have above 500 HP on throttle addiction (NSW) go from listed at 72k drop down to 58k and is still listed on their website.
So if you sell, you'd be safe a 40k I reckon for a reasonably quick sale.