You would have to adjust the fuel pressure regulator to supply less fuel than standard then, so that the amount of fuel served by the injectors is the same as with the 444cc ones.
If you are going to increase injector size, you _have_ to tell the ECU that the injectors are bigger, or it will overfuel.
The amount of time the injectors are open will be the same, but as the 555cc injectors flow more fuel, thus leading to the overfuelling.
...and while we´re at it - I need bigger injectors. Running 600cc injectors at 4 bar, but I´d like to lower the fuel pressure to 3 bar. Anyone have real life experience here? It´s for my 3071´d Pulsar at... let´s say 1.9 bar for argument´s sake. Looks like the 700cc SARD option is discontinued, so perhaps the 800 ones?