Well, If the '15 NISMO is 6.5/10 and the '14 is 10/10 then the 2012 370Z Sport Package is like 3.5/10.
The '15 NISMO is definitely more compliant, I can tell simply by driving the car for 3 hours and not feeling like I have been in a car wreck. I have neck issues (from a car wreck) and I could only take so much of the '14. The '15 is still very much stiffer than the '12 Sport Package though, which I did not think was stiff at all.
I drive about 1.5 hours to a friend's house and back pretty often in the summer and I would always be hurting on the way back in the '14 - I did the same trip twice in one weekend in the '15 with nothing but a smile on my face the whole time.
In actual cornering performance I don't notice much of a difference on the street on fast on-and-off ramps, and even feel like on some corners with bumps the car is less likely to get unsettled. No doubt the prior NISMO would be faster on a perfectly smooth road, though. If you are looking for something to drive at 10/10 then the '14 NISMO would be better/faster. I just wanted something stiffer/sportier than the Sport Package Z (which I liked, but found pretty soft and cushy) but not so bad that I could not drive it for 3 hours.
Anyway yes the '15 NISMO is less stiff and harsh. To what degree, I guess we'll have to agree to disagree. But there is no ******* way it's using the Sport Package springs unless the '13 "Euro Tuned" shocks/springs were way, way stiffer than the 2012.