Could it be a combination of new suspension changes and aero / downforce that would be causing the looseness at high speeds?
This Nismo 2015 review suggests this...
https://www.youtube.com/watch?v=UsrNnW_BYxM
I've driven a number of pre-2015s and love the feel of the car and never felt loose at high speeds, but I have yet to drive a 2015.