Originally Posted by BlazeRex
It would cost subaru nearly nothing to change the location to the downpipe if they wanted to. Its in the uppipe for a reason.
Probably for rapid closed loop changes. The farther the sensor is from the heads, the more delay you get between combustion and measurement. This will effect the behavior of the closed loop system, enough delay and you can start to get oscillation. This has nothing to do with whether or not the measurements are correct though, just the time delay in getting them.