Odd, many Foci owners using SCT soft/hardware are all going to draw through because of bad idle issues with blow through setups. Also you loose anywhere between 20-30% resolution on the same MAF when used in a blow through setup vs a draw through.
A mystery MAF is a bad idea, you should get one that you can get a set transfer function with. Other wise your PCM is going off of incorrect info and can FUBAR your AFR. A hogged out MAF still "thinks" it only flows a certian amount of air. You are taking a risk at flowing more air than the PCM is seeing, which could run the risk of leaning out under higher load, which = BOOM
The lack of a proper tune would explain why you were having issues with the previous MAF. You were running it off of the wrong transfer function. Just an idea for you....
