I was talking to Nareman from Nuvola recently and he said good used GTS are really hard to come buy at the moment, at least at the age / price points he's looking at. Given the often short time the first owners will keep these I guess there'll be a flurry of <1 year-olds on dealer forecourts as soon as the first Sports start being delivered.
I have only spoken with Nareman once and found him to be very impressive so believe what he says.
Half the issue is that the Granturismo works and people are keeping them longer - that's why residuals are at least as good as the 911.
The GT Sport for me does not work as it is not sufficiently different to my car and, if Evo are correct, then it will only be made for two years. If I had a three year rather than one year old GTS then I might see it differently but these cars seem so strong, keeping them until the new one comes out is an easy decision.
2015/16 I look forward to being in the Granturismo replacement.