In the interest of expense I'd look into the various bushings and the like before looking into other items. You can get a lot more twisty fun for the dollar out of some good bushings than out of many other mods.
Not to be contradictory to man show above, I'd steer clear of used springs, as a complete new set is not extremely expensive. That and with something which plays such a pivotal role in your suspension, it's not something I would personally find a good idea to go that route with. I would also take into consideration that if you move to a spring with a higher rate than your current ones, you're going to wear your dampers out faster, which is more money out of pocket.
As far as the control arms go, I belive man show is correct, the STi parts are aluminum vice the WRX steel, this should reduce flex and improve cornering capability.