Everyone that wishes for the option of using a 6 speed is looking for exactly that, the option of using the 6 speed instead of the 5 speed despite the additional weight and possible expense. Nobody has said FFR should have designed the car to accept only the 6 speed.
I don't think anybody has even posted any info regarding clearance issues related to installing the 6 speed, so there is little accurate info regarding what would be necessary to install the 6 speed.