Wow, that does sound like a lousy design to me too. If some small degradation in performance immediately affects the driveability then I’d be ticked off as well. I had one other thought- it would be good to insure that the power supply to the pump is not only delivering the full bus voltage but that it is also capable of delivering the necessary current for the pump to operate properly. The connections near the pump would be most suspect. I’d go so far as to extract the pins from the housing and inspect the crimps. Measuring the voltage directly across the pump leads might reveal if there are any connection integrity issues along the way. Ideally, this should be done with the pump under maximum load. A struggling pump is not good for longevity.
I don’t like the idea of it requiring so much fuel to begin sumping. They may have done that to insure any particulate settling in the tank would be below the pickup screen but that sounds excessive. You sound like a sharp guy and persistant enough to find a solution. I would certainly be interested in hearing what you find and any fix you devise.