The rear spring arrangement on the GD WRX is always quite challenging due to the severe inclination of the strut and the offset of the bottom strut to hub mount. This leads to an uneven load on the spring top and mount.
Its relatively easy to deal with when using stock rubber insulators and relatively light spring rates but becomes more complex with a performance setup with coil-overs. Our spring insulator kit part # K60950 (wrap) will work fine on the top spring but we are also investigating the use of a insulating pad at the top. If succesful we will make these available FOC to any Group 4 owners.
gripgoat, the total spring pack is progressive by virture of the helper spring design but the actual rate is largely linear around the normal working range as the helper spring closes up at normal ride height. This leaves a straight linear main spring which is always preferable in our view.
Hope that covers it.