Some coilovers allow camber adjustments with camber tops, while others do not. Explanation below:
On the 2008+ Impreza's, the rear suspension is a double wishbone type, as such, camber cannot be adjusted via a camber top. This is the same for the BL, BR/BM Legacy/Liberty as well.
Double wishbone type suspension:
Compare this to the macpherson type suspension where camber can be adjusted via top camber plates, and you will comprehend fully what I mean, as the suspension is bolted straight to the hub and camber is changed when the hub angle changes: