THe GCs are just sleeve and custom eibach springs...using factory shock housing, you cant compare it to the TEINs...personally, i think the whole GC thing isnt really up to par...i have used it before so this is coming from experience...
anyway, if you do do real track racing (more than just once a year) JIC is the way to go....the spring rate is ultra stiff, 10kg/8kg, 36mm piston shaft and super duper shock valves...hehe, it's really stiff but it does wonders on the track and, this might sound weird, but it's not relaly uncomfortable on the street either, you feel the road but it doesnt crack your back. hiiiighly recommended if you do lots of track time.