Don't mean to beat a dead horse, but...
YOU CANNOT ADJUST FUEL PRESSURE TO CHANGE THE AIR/FUEL RATIO. YOU CANNOT CHANGE FUEL INJECTORS TO ADJUST AIR/FUEL RATIO.
If you have a properly calibrated MAF sensor to match your fuel injectors, the computer will run at an air/fuel ratio set at the factory.
PERIOD. The only way to alter this would be to get an SVO Extender, a chip, or a programmable computer.
If, however, you are running a combo that requires more fuel than 24# (or 19#) injectors can deliver, you can raise your fuel pressure to allow for more fuel delivery. Your air/fuel ratio will remain the same, though (except for at the high end when the computer will be able to deliver more fuel to richen the air/fuel ratio).
By lowering your fuel pressure to 28, you basically made your 19# injectors into 16# injectors, screwed up the spray pattern, and limited your potential horsepower to something like 204-256hp.
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351W 89 Mustang GT Convertible