To answer your question, the Final Drive is one circular gear that interfaces between the transmission gears and the output shaft. Whatever gear you have selected (1-6), you are still additionally going through the FD before hitting the wheels.
So, getting a shorter FD (like the ITR or the CR-V) will effectively shorten gearing for all forward gears. The combination of a TSX 6th gear comes in because the TSX 6th is taller than the RSX, so you can complement the shorter gearing of your new FD in 6th only (therefore still get good gas mileage and non-mind-numbing engine noise when criuising on the freeway).
Make sense?
_FAQ_