Maybe the resident Subaru Parts person(s) here can find out about purchasing the tranny directly ??
Might not be avaible for a few months-year as Subaru's are selling like crazy, and spare engines and tranny's are in short supply. I was talking to a guy who got a fluke lemon 2012 impreza, and it took over 3 months for them to get a 5-speed tranny to replace his with (clutch came apart and destroyed most of the tranny)! And that was an "emergency need parts now" repair kind of thing, not a "I want to buy a 6-speed because race car!"
I agree however, the 6-speed in previous generations would be cool. Not sure about cost effectiveness, but eh, who cares!