OK, from my understanding the amount of differential pressure between the fuel rail and the discharge nozzle of the injector will affect the overall flowrate..............slightly.

For example, assuming we have a LS1 with a consistent 58PSI supply pressure. The injector flow at 20kPa would be greater than at 100kPa (manifold pressure). Correct?
If this is the case, on FI cars, do we need to modify the IFR to take into account the decreased flow due to pressurizing the intake manifold. I.E. reducing the differential pressure.

Taking this bit of thinking out to the edge of ridiculous, if you were running 58PSI of boost, would your injectors flow at all? Realizing you need quite a larger fuel system to support that boost, but in principle is this correct?

 

You have it all correct. Thank God the injectors will stop flowing at equal pressure or your fuel tank could pop. This is why fuel pressure regulators used on fuel delivery systems with a return line use a vacuum boost reference, this keeps the differential pressure equal. They do not need an IFR table.

 

Unreferenced regulator: pressure difference = (FP + BARO) - MAP
which varies with vacuum (i.e. BARO - MAP) and requires a sloped IFR table.

MAP-referenced regulator: pressure difference = (FP + BARO + MAP) - MAP = FP + BARO
which is constant and requires a flat IFR table.