http://www.ground-control-store.com/pro ... =17/CA=102
These are the ground control coilovers, and these:
http://www.raceperformancemodified.com/ ... ilover-Kit
Are the GAZ coils, which I have and am very happy with.
The ones you linked unfortunately are for an E30, which will not fit your E21. On an E21, the entire coilover system needs to be welded to the stock strut, which is quite a lot of work/complicated. Also, E21s are by far not as popular for modifying as E30 are, and so there are way less companies offering coilovers for them, or any aftermarket products for that matter.
You could go with Bilstein B6's and some racy lowering springs, which will give a really nice ride and a decent lowering.
Or, you could look into what E30 guys do: Shorten the strut, put in Golf MK1 shocks and short lowering springs, or a coilover thread or something. Don't take my word on that though, properly investigate it before you start ordering things:P
My car is about 11cms lower than my friends E21, which is supposedly stock, but I've seen much higher E21s than his, so it might even be more than 15 cms.
If you can find any lowering springs that will go about 60mm or lower, it'll be decently low and handle considerably better.