I am sure someone on here already compared wrx rear brakes to xt6 rear brakes.

 

Will the WRX rear caliper mount to the XT6 rear caliper bracket or will the WRX rear caliper with bracket mount to the XT6 backing plate?  If not, how close is the mounting bolt spacing between the two?

no, they don't fit.

The WRX caliper mount can be adapted to the XT6 (not a bolt on) and once it's mounted you can run the WRX brake components on the XT6 hub. IIRC the center hole is larger and the bolt pattern is different so a spacer to keep it centered and then sort out the mounting holes.

When you say center hole, are you talking about the rotor which slides onto the 56.1mm hub?  Can the rear XT6 rotor be used with the wrx caliper once adapted? 

No, he's talking about the center hole in the backing plate. The GC/GD one is larger... someone (maybe Nico ?) Has done it..

Hmmm, do you need the dust shield from the WRX?  I thought the caliper mounts to the hub, vice backing plate.  In other words, the attachment point for the caliper bracket is part of the hub (1 piece of metal).  So I was thinking that I have to keep the XT6 backing plate and modify that along with the caliper bracket to mate the two.  Then I would have to figure out what rotor works best.  Is this right?  And let's not even get into the e-brake.

 

Borrowed from Nico, in the B/S/T Forums...

 

These are the 2 piston STi rear setup from a GC8 (same rear spindles).

 

Notice how the bracket mounting holes are in the backing plate? This isn't 18 gauge steel. This is like 1/8 thick stuff.

 

On 4WD rear arms (on the EA82), the bearing housing (the sleeve the bearing sits in) has a smaller diameter than that center hole of the backing plate on the GC platform so you have to do something (making a hub centering ring seems the easiest).

But the gap is only 1/8" at most. I modified a set of WRX backing plates to bolt onto my XT6. I ended up slotting two of the original holes in the backing plate to accept the different bolt pattern and then drilled a new hole for the rear bolt with nut. I still plan to reinforce the areas around the "new" holes to make them more precise but haven't had any issues since I converted it to the rear ebrake setup.