perhaps we're confusing lateral stiffness with spring rates here? let me test my thinking - i'm sure someone will correct me if they see it different...
with 100% stiff front springs but plenty of lateral compliance, should get grip on outside tire on even pavement... assuming lateral stiffness difference front to rear is a non-factor, seems to me that spring rate differences front to rear are more apt to impact things like rates of weight transfer fore/aft and rebound and compression rates... slow weight transfer to front would contribute to understeer, and could result from having relatively less compliance in front due to stiffer spring rate - that part i agree with