Yes, your computer will be thinking you still have the stock MAF (65 in GenII's?). If you put it on w/o a chip telling the computer the MAF is larger, it will mess with your A/F ratio. That's about all I know, not too many details.
Well if your car doesn't come with the 80mm MAF then you will need your computer reprogrammed to run correctly. When I had my car I took a SVT Contour MAF which I think its the same as the Duratec one and when I installed it was running good but when I was hitting the accelerator more it used to stall for a second or so and thats why it wasn't programmed for that one. Probably I am wrong but that what I think and maybe someone will chime in if I am wrong.
The only thing you have to do to use is swappin in your stock MAF sensor in that 80mm MAF and take out the old sensor from that MAF then have the PCM reprogrammed to use a larger MAF which in this case is the 80mm MAF.