My car isn't that low, and there's no issue using spacers. You always have the option of custom springs. Eibach used to, and probably still does, make custom springs. IIRC they weren't that expensive.
The principal of an inverted strut is kind of simple, when you think about it. Gas is always on TOP of the oil, right? In a standard strut, that gas is up at the maximum extension of the piston rod (where the piston will never travel). Because of this, standard struts are typically limited in extension but have relatively large compression displacements. The exact opposite occurs in an inverted strut, where the gas is captive up by the strut mount. Inverted dampers can extend quite a ways, but are usually limited on compression strokes. Damper oil viscosity, piston diameter, piston orafice diameter, etc. all play a role as well.
The force required to overcome the effect of the gas in the strut is likely very small between two struts.