I don't know for sure but here are a couple things that I would try
I am in no way saying this will 100% work.
1. Try jacking the front of your car prior to installing the brace. The car may have been in an accident that altered the static geometry of the front suspension. Jacking the front of the car may releive some of the torsion on the frame allowing you to install the bar.
2. Call the TEIN and ask if there is a difference in the RS and WRX bars. I would think that there would be some sort of difference due to the TMIC, however from a structural standpoint the strut towers should be located in the EXACT SAME SPOT on both cars as they are built on the same chassis.