20psi fuel pressure on boosted 02' GT

30 PSI guage seems correct to me. The fact the PCM FP jumps to 40 PSI when the vacuum reference line is disconnected, means the FRPS is seeing the intake vacuum. Remember at idle, the gauge fuel pressure should be less than 40 PSI (subtract the intake vacuum).

Disconnecting the FRPS electrical connector proves little. Other than how the PCM handles a FRPS failure.

IMO, the fact that the fuel pump is capable of producing high pressure means the pump is OK.

What I would look at is a bad/ruptured Fuel rail pressure sensor. BUt to know for sure, monitor the Fuel pump duty cycle. If it's very high, this indicates the fuel pump is maxed out. If very low, that means there is capacity left.

I know that it reads a correct 40 PSI at the PCM. But consider that if the sensor is not giving good values, the only knowledge of fuel pressure is what the FRPS gives. So the PCM will adjust the fuel pump to achieve a 40 PSI reading from the sensor.

On a returnless system, looking at FP doesn't tell you much, the PCM uses pressures drop across the injectors to maintain the target rail absolute pressure. You can log pressure drop with an SCT handheld and LiveLink, usually is 39-40.