Whilst they share the same pads as GTST (and S14, S15, Landcruiser, WRX.......), they are a thicker and stronger caliper developing more clamping force.
Before you spend big bucks on upgrading, do the basics and you will be amazed at the difference:
* change brake fluid for good quality
* check brake hoses and replace with braided (ADR) items
* make sure discs aren't grooved and machine/replace if necessary
AND MOST IMPORTANTLY,
* Use good quality pads
My std GTR brakes have held up to serious track days just by changing out the brake fluid regularly and using good quality pads on the front.
Personally I use Pagid blue, expensive but bloody good