The Wide band 02 bung should never be mounted in the downpipe! Mounting the Wide band 02 sensor in the downpipe will overheat the sensor and cause false readings and can kill the sensor prematurely!!! The highest temperature a 02 should ever see is 900o F. The downpipe can exceed those temps easy (Especially with small turbos.) A little farther down the test pipe is dramatically cooler. The sensor will read the same anywhere in the exhaust stream before any CATS. You just do not want it near the tailpipe as it can get false air at low RPM. ANYWHERE in the test pipe or X pipe (if there are no cats) is going to be exactly the same. The only other concern is to be in the 3 O'clock or 9 O'clock position or higher to keep from having issues with condensation.