The fuel pressure senders are notoriously inaccurate. The actual numerical value of the pressure is meaningless and will vary both in my plane and in other planes. Pressure drop is directly proportional to flow rate. At zero flow rate, there is zero pressure drop. As the flow rate increases, so does the pressure drop. So it is normal that pressure will drop at high RPM. Aren't there gravity fuel systems with about 3 feet of head pressure? That is about 1 psi. So fuel pressure of 2 psi or more is good enough. If the fuel flow rate passes the test described in Van's manuals, there is nothing to worry about. Just compare the pressure with electric pump only to the pressure with both pumps running. If the pressure goes up, then both pumps are working. Like I said, the actual values are meaningless.
An RV-12 owner installed a mechanical fuel pressure gauge below his instrument panel. It required a firewall penetration. And the gauge was plumbed with rubber hose. Even though he knows exactly what the fuel pressure is, I consider my RV-12 safer because there is less of a fire hazard.