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another thing to consider is that adding fuel pump dosent increase flow linearly, ie three pumps dont triple the flow of 1.
i made a 3 pump fuel assembly that bolts in place of a fuel cell filler hat. it has 3 274 ti pumps. a common rail drilled to 13/16(-10 thread drill size) about 2 inches long with 1/4 npt drill size leading to it.
testing on car using my fuel lines( -10 feed -8 return) and regulator set to 40psi i measured the fuel flow returned at the tank. with 1 pump i got about 95% of advertised fuel flow. the second pump added about 80% of advertised. the third added about 60%. the pressure at the fuel rail raised about 2 psi with the 3rd pump. there are 2 things i can think of that contribute to this.

1. flow restriction between pumps and rail that adds to head pressure which makes pumps flow less
2. voltage drop which lowers output.

i measured amperage when i did this. individual pumps drew 20-21 amps. all combined about 55-57. used an old school battery charger to try and keep voltage above 12.5.
final results for me are that at 20lbs boost and 210 injectors fuel pressure is steady to 80%dc at 95%dc fuel pressure is about 4psi down.
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