Well, on the non-speed 3's, the MAFS was notoriously picky about having just the right pipe diameter at the sensor, and just the right placement within that pipe, or else it was not able to correctly meter the airflow. Unlike the MSP, which has it's MAF sensor enclosed in an integral housing, the 3's MAFS just plugs into a hole in the intake piping.
Several intakes had that problem actually... Simota's SRI went through 3 versions just to get things right. I could see improper intake design at, and directly upstream of, the MAFS being a sticky point unless you get it just right.