An STI bar will not fit your WRX unless you get the STI endlinks to go with it. Any bar off a WRX will fit.
I was fairly sure that an STI bar was longer and needed the STI type endlinks that attach to the outer lateral link bolt rather than the link itself. WRX bars have the endlink mount on the outside of the bar, I supposed you might be able to get away with running the endlink on the inside of the bar, but the effective bar rate would go down slightly. If I'm wrong about this someone please correct me. I don't want to spread bad info.