I agree with Martin here, you need to measure it at a given pressure, and for a direct comparison the same pressure. Even then its only a measure of a constant scenario.
Its like speakers, knowing the Wattage (RMS) tells you nothing about how loud it is, you need the sensitivity, and resonante frequency for a start. Then theres the boxes damping effect (pressure) that changes it all over again.
Lifes rarely simple, but then sometime its better to measure something simple well than something complex badly.
Why not fit a fuel pressure monitor to the fuel line, preferably one that records min and max values. Even then its probably Walbro time, but at least with the gauge you might be able to see a difference (less flat spots) and not feel so bad amount the money/grazed knuckles and general petrol smell
