Juan I think you will have a better arguement if the 5spds are unreliable at stock or close to stock power levels.
As it is, if it can be reliable with sub 300whp, 75lbs is certainly significant weight savings and especially at an extremity that would certainly affect weight balance and distribution.
Also, what did you have in mind that could be redesigned and save 75lbs and stick the 818's price goal? Just curious.
I agree that FFR should have considered the 6spd in the design for atleast an option for people like you, but if the trans is reliable with stockish hp, they have nailed an Affordable easily sourced special feeling race car with about as proper of a weight balance as they could manage with affordable Subaru hardware. 6spds are great, but they aren't cheap, they don't typically come in $5k wrecked wrx's, and they are heavier. It's not a hard arguement to understand IF(big if) the 5spd can remain reliable at ~275whp levels.