^ found this topic when researching coilovers myself so thought i'd comment in case someone else finds it.
there appears not to be any kit that are entirely aftermarket in one peice with the bottom spindle etc, they are all using a stock strut cut in some way. GC cut down the standard strut and thread it etc, same with GAZ i believe and KW. KSport / BC Racing are the type that you cut lower down and slip over a bit of your stock strut and get the spindle that way. Whether it is obvious to see that or not, they all basically base their coilovers off stock units. Once tidied up and powder coated or whatever you just can't tell what is what apart from KSport and BC because you see the weld.
Through Tunershop you can get the KW ready to bolt in, through RPM you can get the GAZ kit ready to bolt in, and through Ground Control you can get their own full kit ready to bolt in. KW doesnt mention Core Charge that i've seen through Tunershop, but GC and GAZ both charge a core charge to use struts they have which they cut down and modify into a coilover with a shorter shock inside. GC says you can send back for refund, i dont think GAZ mentioned that.
I am looking at the likes of BC Racing because they are body adjustable on the bottom mount. But then you are still limited in the rear due to the separate spring and shock setup on E30s unless you change to a proper coilover (GAZ offers this option and say all you need is metal top mounts and maybe a strut brace to stiffen things).
I really cannot ascertain clearly what the shock travel is compared between KW/GC Full Kit (cut down struts with short stroke shock) and Ksport / BC Racing. It is also difficult to get solid figures of how much each kit can lower your car. GAZ say -25 to -65 but then they recommend (generally speaking) you convert to full coilover rear which will obviously allow the extra drop up back that most E30s cannot acheive. I am just a bit skeptical that a simple metal upper mount and a strut brace will stop your car tearing itself up over time...
Anyway, as an aside, your other cheaper coil conversion kits are where you basically add a threaded sleeve over your stock shock and wind the springs down. this will lose your shock travel though unless you cut down and put in a shorter insert like G60 Corrado but then you may as well pay for a GC Full Kit to begin with and have them do all the good stuff to it at their factory and it will be all valved properly and painted nicely
Anyway OP Yamahammer - How is your GC Kit???! Pics?