My research has shown me that it uses slightly different internals. Apparently they are made to with stand a little bit more then the GTX. Also, it does run a little more boost. The 323 GTX runs like 6-8psi stock. Most of the 323 tuners tune up to 10psi. Apparently the ECU has a hole in the fuel map at 10.5psi, so they add a chip to get over that. The GTR I think runs 12psi stock. The ECU is already corrected.