Yea i was slightly disappointed it didn't totally stop it from moving, but i would say it does help. Even a new STi in the dealership showroom has this brake pedal movement (but very slightly - less than my 'after' video). I'm assuming that the firewall is already weakened from the damage hence only the slight improvement, but an improvement none the less.
As for not hearing about this, there is a bulletin on this and subaru is aware it is a problem. But there is no recall (probably due to the cost of the repair).