A number of months ago, I put a mechanical fuel pressure gauge on my '93, mounted to the cowl so I could see it when driving.
. At the time, it had my RD2-110 IP on it. So, oversized pump etc.
Put the fuel gauge on the output side of the fuel filter. This setup had a stock mechanical lift pump, stock filter etc.
At idle, it was perhaps 4.5 PSI. WOT 3.5. Never dropped below 3.5 PSI no matter what I did.
That right there is why I like the mechanical pumps. If I need to add a 'primer' pump, so be it. But I'm not getting rid of the mechanical pump.
All I will say is running my E-pump the filter light came on about 70-75 in the hills of PA or any time I was really pushing it while towing. Haven't had any issues with the mechanical one.