I noticed their install page was down for that product. Tip: The rear suspension is VERY similar to that of the GH/GR Impreza.
Here's the install sheet for the Impreza GH/GR rear subframe bushing inserts.
^From Vorshlag's thread on NASIOC; all you do is lower the subframe ~1". I imagine it's as simple as unbolting one side at a time while keeping the middle of the subframe somewhat supported.