I did the master cylinder brace, pitch stop and a few other things for my '12 wrx hatch.
The master cylinder brace I did because of all the stories I head that worried me about the flex cracking the spot welds on the fire wall (there's a thread over at NASOIC, 08+ Subaru WRX Firewall Class Action Lawsuit - NASIOC
about it, a long thread, but some good info in there, I think).
Went for the brace because I could see the master cylinder move if someone stepped on the brake with the car sitting, so I figured the brace couldn't hurt. I was surprised that I could tell the difference in pedal feel with it in, much more solid. Tight space there, so was a little tedious installing, fwiw.
I put in a stiffer pitch stop too, but to be honest, I can't tell a difference since doing so far as feel.
Having a WRX, I did replace the tranny mount with a group N mount - something I think STI's already have?; with that and some stiffer tranny cross member bushings from Turn in Concepts, there's a big difference - No more rubber band feeling when going through the gears, much more solid feel.
There is a bit more noise & vibration, but not bad in my opinion. Makes me think that some of the other drivetrain bushing kits out there might be worth doing too down the road too.