Whenever you change the MAF or fuel injector size, you must get a tune otherwise the ECU is receiving false data from MAF and commanding injectors based on wrong fuel flow.
And from what I've read this does not mean the MAF is bad. It means that the ECU is unable to get the left and right banks within normal fuel/air ratio with the fuel trim.
This can be caused by a vacuum leak, bad HO2 sensors or a bad MAF sensor.
I'd start by looking for a vacuum leak. Listen for any hissing, inspect all the hoses as best you can visually and use propane or carb cleaner sprayed around the vacuum hoses to see if the engine changes RPM. If it does, there's a vacuum leak in that area and must be fixed.
If you have over 100k miles, change your HO2s anyway. They usually last 80-100k before they start getting lazy.
If you know anyone with the hardware and software to data log the ECU PIDs, you can monitor data from the HO2s, MAF, fuel trims, etc. and try to narrow it down. I bought pro software and a decent OBD-II device for around $200 and is well worth the money for situations like this in my opinion.
If interested, here is the device I have and I use the ScanXL Pro software witht he Ford enhancement add-on, although the free software that comes with it should work just fine:
http://www.scantool.net/scan-tools/p...d/obdlink.html