the 324mm gtr discs are a good cheap upgrade but night and day when compared to moving to a larger caliper/pad. Not talking about you, but those who say, "stock brakes are fine" have clearly never driven a skyline with an upgraded brake package; they're just fkn awesome.
check the pad difference, and overall size of the caliper
I always found stock brakes not that responsive; hell, the company car Mazda 2 had sharper brake response than my r33, until I moved to the ATTKD upgrade.
and welcome back ben34, where u been????