I'll try again as the message hasn't shown up
Re: pump life. It can keep vane pump wear down when there are problems. These pumps can be good for 300k+ miles if not abused. During priming it certainly cuts down on dry pump wear. But so can a vacuum pump, which, has the added benefit during priming of showing if the supply system has air leaks in need of repair. The mighty vac is your best friend when it comes to priming and sleuthing.

Re:performance. It can mask a worn pump's issues and prolong drive-ability. So if you're pressure regulator valve is faulty or there are leaky seals it can help with that.

On a good pump, it can amplify the transfer pressures. It will affect fuel volumes as a result. and at low speeds overwhelm the supply pressure regulator and affect the timing advance mechanism. That is why applications designed for supply pumps have internal regulation of pressure in the supply pump. cheap carter/purolator does not. That's the reason the supply pump on the Ford ranger 4d55 is so damn expensive (like $500!), as are other regulated e-pumps. Mechanical lift pumps on the cummins and international 7.3 have internal pressure regulation, and the IPs are calibrated based on the designed supply pressure. in the case of the Stanadyne which has a weaker vane in the pump then the ve style, it's calibrated at 5 psi.

The call for supply pumps comes from the applications of those engines. What is the tank elevation and distance relative to IP. and is it going to vary based on the variety of configurations like long cab and chassis or extended wheelbase. Is there switching that might introduce air.

There will also be more fuel circulation which can help cool a pump, however the drive shaft seal is at risk as is the shaft and bushing, which on these are an expensive repair. I've seen the washing damage to bushing and shaft. The whole supply and return system sees more use and wear.

In an application like these where the pump is calibrated at no supply pressure, of which there are all kinds i.e. all those VW's up to 2003, the addition of the supply pump should not have a lot of flow. But it should also not restrict the flow when the pump in the IP is really sucking.

It's a really good idea to have a pressure/vacuum gauge on the fuel supply between the IP and filter so that the supply situation can be monitored. It can tell you a lot about what's going on including condition of filters. Hope this helps someone.