So it appears that the 1LE is following the "big bar/soft spring" approach relative to the SS by increasing only the bar stiffnesses. Even BB/SS tweaking affects the strut/shock valving, which we don't know anything about anyway.
Sleepy - most of the Mustang lowering springs range between about 25% and 75% stiffer than OE, and that's probably true generally. For the amounts typically lowered, that's not enough stiffer to hold the frequency of bump stop contact down to OE levels. But it is too much stiffer than OE for the OE dampers to properly control for best grip. What stiffer springs with OE damping does do is move the level of "critical damping" down toward "best ride", which may explain why there aren't very many complaints about lowering not fixed by trimming the bump stops.
Geometrically, lowering springs give something up that effectively "steals" part of the benefit that the increased rate is supposed to be providing, assuming that what you're primarily after with your new springs in the first place is more spring rate. Wanna bet that most purchasers of "lowering springs" don't think past the amount lowered?