Well, you already have the #1 (better tires) and #2 (swaybars and end links) arguably biggest suspension improvements covered... maybe you could look into some aftermarket polyurethane bushings to stiffen things up or a relatively inexpensive set of (used?) springs? If you go that route though, you might also have to upgrade your struts too, so that can get sort of expensive with the alighment and installation - unless you do it yourself, etc. You could also look into chassis braces. I think the main difference between WRX and STi control arms is that the WRX ones are steel/heavier and the STi ones are aluminum/lighter/stronger. I guess it'll save you a little weight, but it sounds like it'll be more work than the gains you'll get will be worth. But yeah, it is something else you could do.
On a side note, could you please post a pic of your black WRX with the Rota Torques? I want to see how it looks because I'm still considering getting a set of these too. Thanks!
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)