HPFP is going to most strained at peak torque. Since the HPFP is driven directly off the cams, its flow capacity increases with RPM....so high RPM is generally not an issue for the HPFP. At peak torque, you can get to a point where the hpfp simply can't supply enough fuel per stroke of the hpfp. At high RPM, you might be flowing overall more fuel volume, but the volume required per stroke is less and the piston is moving much faster.
Like you said, you see rail pressure decrease in order to meet flow demands....similar to a water hose scenario where if you decrease pressure you can increase flow. And of course with less rail pressure, you need to increase injector pw
LPFP on the other hand is most strained at peak flow or generally around peak HP or high rpm. In stock form, there is no low side pressure sensor, so we can of have to guess/infer what's going on. If LPFP pressure starts to really tank, you'll get to a point where it can't sufficiently fill the input side of hpfp, and you'll start to see rail pressure drop off in high rpm or other weird stuff at high rpm. The best way would be to monitor low side pressure...MPI kits add this sensor.
In your case, the HPFP is getting maxed out, HPFP upgrade would be wise.
Another issue you run into especially with is38 / e85 setups is running out of injector at high rpm... you get to a point where there simply isn't enough time/injector pw to get enough fuel into the cylinder for a given rail pressure. You can widen the injector window, but if you start spraying too early or late it can result in fuel just getting dumped out the exhaust valve or overall poor atomization/poor combustion etc. For this reason you'll generally see some HP gains on is38/E85 setups with MPI because you're not getting into this poor combustion region etc. DI only seems to work for some setups but not so well for others.
Is your rear O2 sensor still plugged in and/or do you have a P0420 soft code?