Comparing other cars to Subaru's is pointless if for one you don't understand the difference between MAF based fueling and MAP based/speed density fueling and secondly the difference between MAF sensors and what the issue is. The main issue is that the GM/FORD type is housed in it's own enclosure. So for the most part it's calibration stays true. All your doing for the most part is shoving a filter on the end. The newer Subaru MAF as used in all the USDM WRX/STI's has to be calibrated to the ID to whatever housing it is installed in (new intake tube).
GM/FORD type MAF:
Suabru/Toyota type MAF: