... A MAP-sensed car with a new exhaust may run lean enough to actually drop power, because the ECU doesn't know that there's the extra airflow. All it knows is that there's a certain manifold vacuum at a certain rpm with a certain inlet air temp ...
Yep, couple that with a reasonably high flow intake and you be in a mildly lean condition most of the time. When you are cruising your 02 sensors would be able to compensate some, but not totally. The Jeeps ECU is a good example. Its too dumb to compensate for some "power mods"