I doubt that the loss of back pressure would amount to at most a couple horsepower. I'd think the loss of power would be because of restrictions in the cat or muffler and not from lack of back pressure. Is it possible that there is trash in the tailpipe or muffler? Also, I've never heard a properly functioning cat "howl".
If the exhaust is not restricted ... Changing the O2 sensor improved performance. I wonder if there is still some problem related to the reading from the O2 sensor.
I'd be interested in knowing what you eventually find.