Re: XF500 6-speed vs STi
I used to have a glorified Vega with a SBC, four speed, and posi. Turned out in autocrossing it was faster from a dead stop starting in second. I did have to replace the clutch periodically but got very good at it.
CF seems to have a similar issue, a first gear that is too low to be useful and you lose time in the shift. With a rolling start even more so particularly if you have a stock steel flywheel (inertia).
If you have an Android smart phone and Torque, you can measure time to distance which is what you are really concerned about and see for yourself which is faster. Suspect you would be surprised.