Except its not a free flow system.. the injector opens and closes. so when it is closed the pressure stays more consistent.
live life full throttle
When its closed, assuming all injectors are closed at the same time (and there are 8 of them), the whole HPO system has the same pressure because the volume isn't changing and there's no flow (closed system). No flow = no pressure change. Uninteresting situation and not useful for injection. More interesting and useful is maintaining pressure while the volume is changing and there is flow (during injection). This requires sufficient volumetric flow to meet the changing volume needs within the injector, which requires sufficient pumping and minimal loss through restrictions.
Most of the restrictions in the HPO system are in the injector (tiniest holes and passages). Most likely, adding a less significant restriction upstream (the FFD rail product) will not have a dramatic effect on flow. But it shouldn't have a beneficial impact for injection quality. Interested in seeing data that shows otherwise.