The biggest, baddest brakes available to date from Subaru are the STi Brembo brakes, using a mammoth monoblock four-piston aluminum caliper in the front, along with a huge 12.7" rotors. Most late-model Subaru cars share the same front brake mounting, so you can bolt up these front brakes to almost any 2002+ Impreza, Forester, or Legacy. Keep in mind you'll need wheels to clear these large brakes, which means at least a 17" wheel. And just having a 17" wheel doesn't guarantee fitment either, because the wheel needs to have enough spoke clearance to fit over the caliper. MOST, but not all, of the wheels we sell will fit over Brembo front calipers.
Note that only 2004 WRX STi rotors are going to bolt to the 2002-2014 WRX cars, because they share the 5x100 bolt pattern. 2005+ STIs have a 5x114.3 bolt pattern, and the rotors will not bolt up to the 5x100 cars. (2015+ WRX have 5x114 hubs.)
The REAR STi Brembos will NOT bolt to 2002-05 cars easily, for two reasons. One is that there's a caliper mounting bracket that is different. And even if you had a bracket that would allow you to hang the caliper on, the inner diameter of the STi rotor is larger than that of the other cars, so the parking brake that fits in there would not reach the rotor. The rear Brembo adapter kit we sell comes with both the caliper mounting brackets AND special rear rotors that are large on the outside, and smaller on the inside to fit the non-STi parking brake.
The 2006-2007 WRX came with four-piston front/two-piston rear brake calipers, and the gold Brembo calipers WILL bolt directly to the front AND rear of that car without adapter brackets. You'll still need the special adapter rear rotors, though.
For 2008+ WRX models, ANY front Brembo caliper (older gold color or newer black color) will bolt to the front of the car. You'll need 5x100 drilled STI front rotors to fit. On the rear of the 2008-2014 WRX, the older gold rear Brembo caliper will NOT fit directly, but we have a bracket kit that will allow it to fit. The newer 2008-2014 black STI rear Brembo caliper WILL fit directly, without an adapter bracket. Again, you'll need a specially drilled STI rotor to bolt on with the caliper. 2015+ WRX cars are the same as 2014 in terms of caliper fitment, but the rotors are 5x114 bolt pattern.