there are no official KYB AGXs for the WRX yet. The only ones that will fit are the ones for the GC8. itll fit but you will have some camber problems in the back. As for handling and performance, it seems pretty good. in SCC article like couple month ago, a wrx w/ h&r and agx handled better than a wrx w/ full sym suspension.
Call SPO, but I believe the only "modification" are included zipties or something similar to attach the brake lines as the GC model come w/ no brake line bracket. I think these could be a viable option, however after many requests on the i-club I haven't had any comment on the exact amount of increased negative camber. There is someone who said he will post his precorrected alignment specs as soon as he installs them (he is waiting for STi tops). With the OE spec alignment already set up for plenty (read too much) of understeer, camber at a wopping -1.3 degrees in the rear, compared to only -.25 up front, you wouldn't want too much of an increase as most camber bolts are good for only 1 to 1.5 degrees of correction.
I would consider these struts (I am myself) only after finding out exactly what increases in neg camber are expected- including the addition of lowering springs and how much correction you can make.