Skunk2s are "harsher" than GCs because most people with Ground Controls buy the groupbuy Limited Editions, which have VERY conservative springs rates. Skunk2s on the other hand have more performance oriented rates, which don't match well with the cheap shocks many people put them on.

AFAIK, Skunk2 sleeves also use Eibach springs. There are only three companies in the world that make their own springs.