First, I would not recommend the Kinetix plenum. If you are going to get a plenum, get the Crawford cast aluminum plenum.
As for plenum vs. 6MT final drive, I cannot say for certain. My gut tells me the final drive may be better overall. However, keep in mind that you will need an entire pumpkin from an 6MT. There is enough differences in the pumpkin that it only makes sense to get the whole thing. This will significantly raise the price of the mod. Even with a discount, the new pumpkin will cost twice as much as the plenum.