I don't have the legacy RV-12, so I can't address any specifics.. but there has been a few service bulletins regarding fuel pressure, which you may already be familiar with...
If you're not sensing any change in performance/RPM..and since you say that the pumps have already been changed in an attempt to fix the problem.. then perhaps its just a sensor problem? According to the RV12 G3X electrical diagram, you have a Kavlico 101690-000 sensor.. which only measures up to 15psi...
Does the G3X need to know which sensor you have? I assume the Van's RV12 G3X default settings file will load that in automatically. (my rationale here is that the g3x needs to know that 0-12v maps to in terms of psi)
You can also try verifying that you have good electrical connections between the 3 wires at the fuel pressure sensor and the 3 pins at the GEA24 (apparently pins 1 2 and 4 of the largest D-sub connector).
You can also try just swapping out the sensor.