best thing to do is google 's15 specs', 'silvia info' and stuff like that...i didn, had like 20 files of shyte telling me everything they had improved, like the helical and viscous suspension and stuff... the jdm are more powerful coz they have less restriction, ie exhaust and stuff (i think)aus have 147kw (same as a vt commo:P) and i think the jdm have around 175-180kw
gt is luxury model with loads of extras (as has been said) but most people just get the spec r (lil heavier than spec s) and remove the spoiler n stuff to make it look more slick so to speak
so basically, google it mate, google knows all!