That's not quite what I'm saying. Remember the stock MAF sensor maxxes out at 5v. Now say you're flowing 300g/s through the stock MAF housing and hitting 4.9v. This means you can't flow any more air and still get an accurate measurment through the sensor. On an STI you add a bigger MAF housing and re-scale the intake calibration appropriately. Now you will be flowing the same 300g/s but at a lower voltage... 4.4v for example. With this housing and scaling, you could flow more air and still stay withing the 5v limit of the sensor. So lets say with the new scaling you can flow 500g/s which would register close to 5v. This is good and fine on the STI ECU because it can reference flow all the way up to 500g/s. The problem with the WRX ECU is that it won't take any flow values above 300g/s. So if you install a big MAF housing and re-calibrate the intake map, you will be hitting the maximum 300g/s at around 4.4v. But since the ECU only registers a maximum of 300g/s, the voltage range you gained with the big MAF becomes useless. Once you hit 300g/s, the rest of the intake scaling map becomes flat, with 300g/s from 4.4v all the way to 5v. This obviously is not a limitation of the sensor because it can in fact go to 5v, but the ECU simply can't use any flow rate above 300g/s.

I know this is a bit confusing, so let me know if I can clear anything up better.

Thanks