TB993tt I believe the reason for this is that for security reasons the ecu may detect all parametres/problems almost instantly (as you describe) but then adjust parameters only gradually.. This probably has to do with minimising the risk that sth goes wrong and the reliability of the engine is compromised.
Having said that, the effect of the plenum on airflow should not require such big adjustments (as per my experience above) and a couple of spirited drives should do the trick. However one should consider whether the stock ecu's parameter tolerances can fully capture whatever benefit the bigger plenum would introduce or one would need an adapted ecu software to capture it..