As I understand it the 6 speed box is normally supplied together with a 3.62 diff and the 5 speed with a 3.92. This means that the difference between the top gears is not as much as is made out and although 4th in the 5 speed is a direct drive, the same as 6th in the 6 speed, the actual mph/rpm in those resepective gears will not be the same due to the different diffs.
Bunging this stuff into gearcalc with 195/55R15 tyres gives:
5speed:
21.56 mph per 1000rpm (70mph = 3246rpm)
6speed:
19.26 mph per 1000rpm (70mph = 3634rpm)
If you fit a 5 speed with a 3.62 diff then you get 23.35 mph per 1000 rpm and 70mph = 2998rpm (and a theoretical top speed of 175mph ).
I suggest that you a) play around with gearcalc (search on here for a link to a copy) and b) get a ride in a 5 speed box car and see if it really does make that much of a difference to you. IMHO it is wind noise that dominates once you reach motorway speeds (and I have a 6 speed, high power K and no carpets or any other form of sound deadening).
Yellow SL #32