Here's a link
to another good thread on this topic. Toward the bottom of the first page, legacy777 posted a scan from the service manual. It summarizes the various Impreza braking systems; it shows various models having drums vs discs, or different size rear discs. However all models appear to share the same Brake Booster, MC, and Proportioning Valve, with the same split point of 285 psi and reducing ratio of 0.3. So more evidence that perhaps the New Age TS and OBS already have the correct MC for the swap. As I mentioned in an earlier thread, Subaru may find it simpler and cheaper to use one set of parts across all models, and adjust for bias by just sizeing the drum piston accordingly. This way we would get the same amount and pressure of fluid going to the rear brakes, and get the same or similar amount of ultimate rear brake torque.