student of subie brakes might be more appropriate. As we both know, you have some answers to some interesting rear brake questions on the way to you now
. Can't wait to hear them.
As far as this upgrade goes CRAZY KEN, let me tell you what I know and don't know.
Let's talk about Subaru rear disc brakes.
The rear rotor used on my Legacy(1994,) all RSs, most Legacys and Imprezas, all the way up to and including current WRXs, is a 266x10mm solid rear rotor utilizing a 170mm internal parking brake. The only 2 other US rear discs offered since 1990 were the 266x18mm vented rear disc found on BC/F Turbo Legacys and the 290x10mm solid rear rotor found on ALL 2000+ Legacys(not just the H6 equipped ones) with rear discs. Both of the these discs were also equipped with a 170mm parking brake. The reason I am mentioning the parking brake size is because a few JDM WRX-STis came equipped with 290mm and 316mm rotors with a 190mm internal parking brake.
Another rotor, which is the one I believe Ciper is alluding to, of significant importance is the 290x18mm(with 170mm parking brake) vented rotor which comes with the $1300 2pot rears you see some vendors selling. This rotor has never been sold on a US Subaru.
The upgrade to the 266x18mm vented rotor can be found in detail on Brian's(North Ursalia) northursalia.com. It requires purchasing the turbo Legacys caliper/brackets and rotors but will use your stock backing plate/dust shield with no mods and the stock parking brake. It can be a bit expensive to pull off because it does require so many components to be bought.
Now in reference to the "H6" upgrade, here's where things get a little confusing. If you have any 2000 and up Subaru with rear discs, I am sure of the following things. To upgrade to the 290x10mm H6 rotor you need only to buy the rotors, larger caliper brackets and either remove or bend the rear dust shield(thin sheet metal shield that is attached to the perimeter of the backing plate.) Every thing bolts right up and uses your existing parking brake internals. A sweet and inexpensive upgrade. Part numbers are listed in the thread linked above.
Now here's where the confusion comes in. When dealing with 99 and older Subarus. At some point (and I continue to try to find this info out for sure) in 98 or 99 Subaru changed the shape of the caliper/bracket on the single pot rears. So this leads to the question, will the old caliper fit in the "new style" caliper bracket and therefore the larger "new style" caliper bracket used with H6 rotor. No one has definitively answered this. Worse case scenario, as Ciper said, is guys with the older style caliper/brackets may have to replace the calipers as well as the brackets and rotors. If you are willing to wait a few more weeks for an answer I will have a definitive one for you. I am pursuing a rear brake upgrade and info on a daily basis at this point.