hm.. interesting... can heavier spring rate induce more oversteer?
In a AWD system like GTR, an understeer is natural behaviour when pushed beyond its limit, characteristic of an 4WD/AWD car. When combined with stiffer front spring rate, the understeer effect can get worst.
Maybe Nissan engineer decided to put stiffer rear spring rate and softer front spring rate to dial out some of those understeer effect and to help the AWD GTR to turn in more into the corner, all in order to achieve a more balanced chassis dynamic.
This setup would be different in a RWD GTS-t because of RWD tendency to oversteer when cornering hard under power. That's how I guess on GTS-t the rear spring rate is softer than the front...
I'm no expert, so if it doesnt logic enough, feel free to discuss...
just my 2c....