6 speed manual transmissions are maily made with really close ratios
example: my cougar vs. my cousins '08 civic SI.
driving 75mph on the highway both of our car are at the same exact rpm. but im in 5th gear and he is in 6th. so he has to shift alot more.
also, many times 6 speeds are put in a car that has no torque (just like the civic). the reason is so that the driver is able to keep the rpm's higher while driving, where there is more power.
its definately unnecessary, and not only that, but i would say completly stupid to hook up to a 3.0 liter v6 that has PLENTY of torque to a 6speed. unless the ratios are the same or at least longer which i doubt
EDIT: btw this was written without any research done on that transmission