I am a little confused on your results that the 8045 prop pulls the same RPM`s as the 9 inch prop?
I use the current (amps) measurement from the ESC telemetry and verify it measuring the voltage drop across the 50mOhm resistor inline with the power input. I'm not sure how you'd estimate it apart from using eCalc's PropCalc. For what it is worth, PropCalc says the 8" should run faster (9816RPM) and pull less current at max (8.9A). I've also tried with two different props from the same bag. All props are balanced before testing too.