I'm not refering to the strut tower brace, but the metal one that seems to bolt onto the back of the trunk wall, right beneath the spoiler. I saw it in an ITR before, and was curious, does it make a difference? What purpose does it have? And where can I find it?

 

Yes, it does make a difference.

No, you can't just bolt on to a GSR, LS or RS. You'll need to weld into these models with preferrably reinforcement plates.