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i'm not sure what your mechanical setup is, so i can't really say for sure why your MAF required adjustment, but i know that even on stock cars some have found it to be necessary.
there are a few major components to fueling calculations that effect what you see in fuel trims or fueling error charts if you use them. MAF scaling, port injector scaling, injector battery offset (or dead time, or latency, whatever), and also direct injector flow rate. generally speaking you adjust what you change, so if you install an intake, you look to make maf corrections. if you install larger injectors, you're adjustments should be to the scaler and latency, not touching the other areas. the only time in which you'd have to mess with direct injector flow rate is when you change fuel density (use e85).
when you find yourself applying a flat correction to the whole maf scale, that is a smell. it tells you there is something else wrong, unless of course you've just changed the diameter of your maf tube.
it's very important when trying to calculate or correct fueling error that you maintain process separation between the involved components. i've found logging port and direct injection independently (through port/di ratio adjustments and logging injection mode) to be absolutely crucial in getting everything correct once e85 was introduced. if aftermarket port injectors are involved, you'll want to isolate them as well and eliminate noise introduced by the direct injection system when tuning them.
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