I'm using R32 GT-R calipers on the S15 with DBA 4000 slotted rotors and Ferodo DS2500. Sure there was an improvement from the standard S15 calipers, but by no means a cheap exercise eg. calipers, rotors, fluid, lines all add up.
The difference in R33 GTS-T calipers and R32 GT-R calipers are that the GT-R take slightly thicker rotos 32mm instead of 30mm.
Go with better pads and brake fluid like the others recommend and you should see some difference.
IMHO swapping the calipers would not yield you that much, if at all.
C.