There are absolutely zero "performance" differences between the 06-07 WRX and WRX-TR.... none whatsoever.
The differences are:
1) WRXs come with paint-matched mirrors, TRs are black
2) WRXs came with 1-piece seats, TRs have exact same bolsters, but 2 pieces (with the separate headrests)
3) WRXs have aerodynamic sideskirts, TRs are just straight
4) WRXs have automatic climate control, and a double din CD changer stereo, TRs have basic manual HVAC controlls and a single din unit.
5) WRXs have better floormats, and aluminum pedals
Engine and suspension are 100% the exact same, as is everything else on the car. I bought a TR to save about $2000 - $2500... I think it was worth it since I kept a modest looking exterior, but made all my changes under the hood or inside the car.