couple of things to consider:
how is your ECM measuring airflow? MAF or speed-density?
if you alter the VE via intake changes so that less air is entering the engine for a given RPM and manifold pressure, the MAF system will adapt, since it only measures airmass, but a speed-density engine would actually run richer due to the ECM thinking more air is entering the engine than reality. the O2 sensor will allow for it to adapt eventually, but until then, you'll actually lose fuel efficiency.