As Mrkyle3 stated above, with upgraded suspension the late model WRX and STI feel almost exactly the same on a track.
The STi does not have some magical properties due to the red STI emblem, nor the obvious performance increases it did in previous years. Its a WRX with LSD, 6 spd, bigger wheel bearings, brembos, all black interior, and better factory suspension and TMIC. The engine internal differences are so small as to be completely irrelevant. This has been proven many times on the recent models.
Most of us know the differences between the cars I stated above, yet these threads continue to fester because some people think that red STi badge adds 40hp .... It doesn't. Its been proven several times that the two models dyno the same, and have an almost identical power curve. Some attribute it to more drivetrain loss in the STi, others to the WRX being under rated. Either way, they are about the same in the power department.