There are two versions (mechanically at least) of the ITR. The one we got is the same as the '96-'97 jap-spec model. In '98 they came out with the Integra Type-R 98spec which has 16" wheels, thicker roll bars and the 2.5" exhaust system.

Anyway.

The cat on the '94-'99(?) GSR is shorter than the RS/LS/GS/SE. So the collector is longer. The '00-'01 GSR has the same length cat as the non VTEC models. If you have a "universal" cat it still needs to be the right length to match the header.

When people have crazy all-motor setups it makes sense to step up to a 2.5" exhaust, and the "JDM ITR" type headers have a 2.5" collector which helps in that regard.

Sorry if I sound repetitive, just wanted to cover all the points.