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We also upgraded the WRX's rear brakes with Stoptech's offering. The front and rear brake kits are designed to work together and complement each other even though the car will work fine with the stock rear and Stoptech front kit. The kits are validated to work well either way for reducing stopping distance and compatibility with ABS and stability control. 
The rear brakes use Stoptech's ST22 caliper. The ST22 is a lightweight two piston fixed caliper with a stiff forged aluminum body and radial mounting. It is one pound lighter per side than the stock one piston sliding caliper. 
The ST22 caliper, like the ST40, has stainless steel abutments at the ends of the brake pad backing plates to prevent the brake pads from digging into the caliper body and sticking during long term use. The ST22 caliper also has high temperature silicone dust boots instead of the usual rubber. The pad retaining clip is spring pre-loaded to make the pads a little less likely to rattle and squeak.
Stoptech's rear rotor has all the same features as the front Aerovane rotor. It is much bigger than the stock rear rotor at 345mm x28mm vs the stock 286mm x8.5mm. Despite being much bigger it is 5 lbs per rotor lighter than stock due to the alloy mounting hat. 
The rear rotor is full floating like the front rotor. The hat is machined to be the internal drum for the rear parking brake. This is one of the reasons why it is so much lighter. The parking brake drum is now aluminum so although it is perfectly fine for a parking brake, we would not recommend using the parking brake for a drift brake. 
The rear brake pads are Stoptech's street compound just like the fronts. The pad area is about 30% bigger than stock. Adapter brackets and braided steel brake lines are used for the rear brakes as well. 
