The 24lb/hr injectors are too much for the combo, but unless something else is setup wrong, they won't kill fuel economy like that. The computer will adjust the injection cycle with information from the MAF and it's database. I've seen a bone stock (well, K&N filter) 1987 SD Mustang GT run great on 24lbers. No fuel economy loss. Still got near 30mpg on the highway with 2.73's.
Since you replaced the O2 sensors, and the MAF is calibrated for 24lb/hr injectors, I don't see those as being problems. The thermostat should be just fine for your application.
The other possibilities that I see are: BAP (Barometic absolute pressure sensor), if it's having major issues or more likely, a vacuum leak, it can cause very poor fuel economy and drivability, the temperature sensor (which could be telling the computer the temp is actually -20* or something if it's malfunctioning), the computer itself not going into closed loop, or a vacuum leak somewhere else.
Have you done a tuneup at all?
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