It's not the max fuel volume of the plunger at all. It's the effect of the fuel volume transfered at a given meetering sleeve position. And the amplification of those psi.
I agree it's nice to pump fuel to the IP quick. doing it myself by times. Can prevent a lot of wear due to cranking dry. and restore supply pressure when getting creative with switching fuels.

Using them to satisfy a thirst for fuel volumes sort of defeats the advantage of the diesel imo. lol
It's about a pump's calibration specs. I can't tell you why a specific pump application is set up on the test bench to achieve it's fuel volumes and efficiency based on the application's supply pressure. How far away are the tanks? Are there pre and post filtration? Is there a lot of elevation to gain? Did it intend to get run with a supply pump, because it needed it?
Presumably the ranger with the e-lift pump is calibrated differently then the pump in the L200/mighty max and my delica - all kinds of the ve style pumps were fitted OEM without lift pumps.. The high price of that ford lift pump must reflect it's internal regulation - a key consideration. they want like $500 for that ford pump (which cross applies with all kinds of gas engines oddly enough), compared to the relatively cheap in line pumps. Mechanical lift pumps on a cummins with a ve are internally regulated. The stanadyne db2 which has a relatively weak in comparison vane pump assembly, is calibrated at a 5psi supply pressure on the 7.3
Calibration wise, the transfer pressures are at the root of it. but I appreciate at a certain point in it's life cycle, calibration gets tossed out the window for one reason or another.

While it could prolong vane pump life, and mask other supply problems like sending unit and fuel line issues, etc, and improve performance on a worn pump, it also affects the injection timing. I think it could have a negative effect on efficiency, for one, because transfer pressures rely on venting to the housing, which may be be troubled. It may cause or exacerbate leaks at the shaft seal and elsewhere. increased cycling causes fuel washing at the same time as it cools the pump. 200k mile pumps can have noticeable drive shaft bushing wear from constant leaking past the seal. The things are so dynamic it's hard to say where the +/- balance is. Cost of repair being part.

Without a complete reseal or even troubleshooting the IP an epump can make a big difference in driveability with a worn IP because it will advance injection timing, either by boosting the transfer pressure and or acting on the timer. It can mask a bad supply pressure regulator. A better approach is to advance the pump's timing manually. And inspect the IP regulator for bad orings and loss of function, perhaps even setting it higher. it's today's fuels and the fact many of us have less then ideal pumps and fuel supply. Does your truck run better when you've a jug of ATF in the tank? It's both the viscosity and the sealing aspect of it. Does it start and run better with a e-pump? it's likely because it overcomes the supply/transfer struggles.

The internal pressure of the pump (transfer) is going to exceed the supply considerably of course, but that doesn't have anything to do with the spill of transfer pressures back to housing and fuel metering.

I've been told the very best piece of equipment for priming and sluething supply system is the mighty vac a hand held vacuum pump which not only gets the fuel there but tells you if it will hold there (esp. if it has a gauge).