I had a Magnafuel 270 pump on my old pump gas Duster that I couldn't jet up the six pack enough to slow it down in the 1/4 mile so I bought a Magnafuel 300 pump to replace the 270.
I flow tested the 270 with all three fuel lines into a 5 gallon plastic pail three times for 11 seconds, it flow within 3 ounces on all three test, I then replace that pump with the 300 and reflow it three times, it flowed exactly the same amount as the 275
I called Magnafuel and told them what and how I did the two tests and they told me by allowing the fuel to flow out of the lines with no restriction I wasn't loading either pump to its full capacity, They said to put restrictors in the lines and retest the pumps. I hook one line to a carb and that stupid(300 pump) flow almost 50 % more out of two lines
I ended up finding that I had screwed up the system by using a BG 500 10 micron fuel filter between the pump and the tank, I didn't know that all paper element fuel filters are rated at 10 microns
Many ,many gremlins out there
Have fun Gus, let us know what you learn and find