mikes correct, the "perch" height is different... and I beleive the 48mm will give you a lower ride height also ? CSi Inc. quote $1200 each for this beauties! ooch !... thats if they actually have them in stock ?
I have the CN springs/bilsten set up which drives just great , but would have likes a slighlty lower ride height (about 1/2 an inch lower coincidently !!) I stayed with 50mm to be OEM, and also Carl says if you go too low it starts to muck up geometry/strains components etc ...unless you go to coil overs