Originally Posted by aajami
I'm the guy who did the ZCP conversion.
I don't think there's clear consensus on what the difference between the struts are, primarily because there are no obvious visual differences between the two. My belief is that their valving and compression rates are a bit different based solely on the fact that they have different part numbers and different prices. It was also interesting to note that the ZCP struts are the same ones found on the 1M and the standard Japanese market M3.
As for the lowering, there's also no consensus there. The only difference in part numbers between ZCP and non-ZCP cars are the struts and springs -- the spring pads are the same part. It's either done through the springs or the spring perches (though as some have noted, the difference in perch location is practically imperceptible between the two struts).
Why not just order ZCP springs and see if you get the drop. As I recall, they're cheaper than the Dinan Stage I kit. If you don't get the drop, then you can either return the springs or buy the struts. Either way, you'd confirm where the drop comes from.
So there is a slight difference in perch locations?