The ITR tranny has an identical final drive ratio of 1: 4.400.
Fifth gear is a teensy bit taller in the ITR (0.848) compared to GSR (0.787)
I'm awful at math, but I would guesstimate that at 80mph, you're closer to 4500 rpm.

h:
And we do have an ITR forum....you could always ask
them.
BTW, I got my numbers from
this thread. You might want to check it out sometime.