After conducting an additional flow test with the stock S4 MAF and a different power supply, to confirm the results obtained, I swapped to an 85mm EPL MAF for the S4.
![Stock S4 Hitachi MAF](https://myaudis4.com/wp-content/uploads/stock_s4_maf.jpg)
![EPL 85mm S4 MAF](https://myaudis4.com/wp-content/uploads/epl_s4_maf.jpg)
To my surprise the resulting airflow for each voltage point that I measured corresponded closely with the readings obtained with the stock diameter MAF housing.
![Stock vs EPL MAF housing](https://myaudis4.com/wp-content/uploads/stock_vs_epl_maf_flow.png)
Added a comparison with the MAF attached to the stock modified airbox.