The ECU has a "map" program for both fuel and ignition timing. Each map is based on RPM and MAP sensor voltage (MAP = manifold absolute pressure, sort of how much air is going into the motor). So for any given combination of MAP voltage and RPM, the ECU has a stored value of injector duty cycle. This is the signal sent to the injectors which tells them how long to stay open. The injectors always flow the same amount so the way you change how much fuel is injected is to change how long it is spraying fuel. Some of the time, the ECU uses the oxygen sensor to adjust the base duty cycle values but that's not all the time.

So what happens when you just change the injectors to a higher flow rate, is that the ECU doesn't know this and sends the same duty cycle signal to the injectors. But since the injectors flow a lot more fuel for the same given duty cycle compared to the stock injectors, you run way rich. Also since the ECU does not actually read the oxygen sensor most of the time, it doesn't compensate for the rich condition. What you need to do is get a lower duty cycle signal sent to the injectors so they get the right amount of fuel into the motor.

One other thing you have to deal with is preventing the ECU from throwing a check engine light from boost. The MAP sensor itself is capable of reading up to about 12 psi of boost, but if the ECU sees a voltage indicating any sort of boost it goes into a limp mode and throws a check engine light because the duty cycle table does not include MAP voltages that indicate boost.

What people typically do with an AFC is run 450 cc/min injectors and set it to lower duty cycle by 40% across the board. The AFC then reduces the MAP voltage by 40% so the ECU doesn't think it's reading boost. It then sends a duty cycle signal to the injectors that is 40% less than is actually needed. Since the higher flow rate injectors flow more fuel for the same given duty cycle compared to stock injectors, they automatically "multiply" the duty cycle signal to get the right amount for the actual amount of air going into the motor.

With 550 cc/min injectors, that is too much of an increase over stock for the AFC to compensate for. So you need to get a replacement ECU where you can change the actual duty cycle table itself, instead of use a device external to the ECU to trick it into giving you the duty cycle that you want.

If you need any sort of clarification let me know.