ITR rear suspension is different than GSR. It uses a split lower control arm with an internal shock mount. It resists flex much better. Also ITR uses larger hubs and bearings - big advantage in longevity on the track.

Finally having 5 rather than 4 lugs is a strength thing as well.

If I were you I would decide what I wanted to do to make my car better in performance and not try to turn it into a Type R.