on the pump there is a schrader valve for testing the supply from the fuel tank , what happens to that when you rev it to 4000 rpm ?

from memory 4-5 bar is normal

if the pumps good the only place for the injection pump pressure to go is back through the fuel pressure regulator on the end of the rail.

long term of plus 30 says it not going out of a injector.

injection duration will get longer as the PCM tries to richen up the mixture.

we had one of these a few years back no high side pressure , it was a bad pump, we put a new one on from ECP and it was still the same , so we got another one from ECP its was still the same , we tested every thing under the sun , it drove me mad, so i found a guy on eBay that rebuilds them, the pump came back and the car was fixed, for a few days and then the symptoms were like yours.
sent the pump back to him and once it came the car stayed fixed.

The ECP pump thing was maddening, i think that people buy them swop the bad bits out and send them back to ECP. as there a expensive pump .