Originally Posted by foreigneRS

mass flow is mass flow. it cannot change unless you have a leak. the volume flow changes depending on density (temperature), but the mass flow stays the same.

provided the maps and run programs are sophisticated enough, they should cope with a change like a FMIC. i'm not saying that they are sophisticated enough on a subaru of that age though, so maybe the 'tuner' knows that they are not and is giving good advice, i don't know.