I'm sure someone somewhere has a rear axle dyno and that would be the way to answer the question. I've never seen a rear axle dyno but if someone asked me to build one I'd use an electric motor to turn the pinion. Then I could tell how much power it takes to spin the axle at X RPM. It would be fairly easy to test different rearends since all you would need to do is attach a driveshaft to the yoke and flip the switch.