unless you plan to do some serious track work with the car, as stated, the stock calipers and rotors should be fine with lines, pads and fluid. For those wanting to shave weight, switch to a floating rotors (I know the fronts already exist for this size but not sure on rears) and maybe snag the Subaru (not brembo) 4/2 pot setup though the rear would require a bracket (simple).
For a Brembo 4/2 pot setup, you are carrying more weight. It's sprung weight and rotational as well. So worse suspension performance and slower acceleration....to what extent, eh, I don't feel like working up the rough math
Having owned a 94 Impreza with a WRX swap that was fairly comprable to the BRZ (but made about 60% more power at the time of being RWD), base WRX brakes with pads and fluid will perform plenty well.
As stated above, those needing larger rotors for heat management, yes, the LGT would be the next step (and it needs the caliper bracket too IIRC) but I would get the BDA 5000 rotor for weight. I'd also do the same (or the Giro Disc or Performance Friction rotors) if going to the 'STi Brembos'....weight, save it where you can, without sacrifice.