Im always open to suggestion and the way your putting it does make it sound feasible, im going by exactly what ive seen in my own personal cars, unless they had slipping transmissions, screwed tacho's or different diff gears im still sticking with my theory.
If the v6 and v8 t5 manuals revved at exactly 1800rpm like dan said and share a differant final drive ratio how can the crank possibly be spinning at the same speed? in the autos where the final drive is identical ive noted my claims and thats why i believe im right. There cant be any way possible i got 5 vn v6/v8's with inaccurate tacho's..
seeing as you are open to suggestions, i'll type it again. both the V6 and V8 vehicles equiped with the T5 trans have the same final drive ratio. and as others have stated the same rpms at 100km/h (1800rpm, 0.72:1 5th gear and 3.08 diff ratio's).
i have attached a simple spread sheet that shows the final drive ratio's for a T5 equiped car with 3.08 diff ratio and fitted with 17" wheels. in the last 2 columns i've added the OD ratio for the 4l60 auto box (0.70) and the speed difference between the T5 final ratio and 4l60 final ratio.
you will note that 100km/h@1800rpm is about the same as 110km/h@2000rpm (i.e. increase in RPM = increase in road speed)
the only other variance to allow for possibly is different characteristics of the stall converters fitted to the transmissions. if the V6 converter is "looser" then the V8 model then i would expect to see a slight increase in RPM for the V6, however i don't think the stall speed of the V6 converter is higher then the V8. i could be wrong.