you really don't want springs that are
that stiff. I would probably go 150 lb/in
maybe 175?
you definitely don't want lowering springs. the entire reason people lift their cars is to get more suspension travel. your car has plenty of clearance for rallycross. Suspension travel sucks on our cars, you don't want to loose any of it.
you can find used coilover sleeves for dirt cheap. I would just get a set of those and run some giant 2.5" springs.
Most impreza guys run 225F/200R with no sway bars and their cars are much heavier.
Here is a set an impreza guy made out of stock STi struts: