h&r and eibach do do a good amount of research. maybe what i like best is that they prototype their parts on cars in the u.s. before releasing it here, and sometimes make adjustments based on those results.

tuners do specify their own spring rates, as well as specially valved shocks. when used together, they produce whichever desired ride/handling characteristics the tuner intended. in general, matched spring/shock sets work pretty good whether it be from tuners (i.e. brabus kit) or suspension specialists (i.e. h&r cup kit).

it would be interesting to see a chart of spring rates for all the tuners vs h&r and eibach, to see exactly how different they are.