I have lots of data from the Eagle Racing MD2 dyno if that helps anyone wanting to build a dyno. What's needed to make the MD2 dyno better is higher sample rate (it's only 50hz) and more accurate current/voltage measurement. If the sample rate was increased to say 500 hz with a larger flywheel you would really have something.

I really like the idea of a geared dyno. At least it keeps the flywheel RPM down and if you are using it to do your own comparisons errors due to gear friction would be irrelevant since you would use the same value for all tests. Apple to apple. I would expect the loss to be less than 5% for a single gear mesh but that's just a guess. A 1:1 drive with a much larger flywheel would also be good but the RPM for a low turn motor with boost would be terrifying. The MD2 dyno shuts off at 30000 RPM and that's a good thing IMO.

One thing to note is the torque output of a brushless motor is not linear particularly at higher timing settings. I have spent lots of time researching modeling DC Motors and you can get good agreement with a simple model for brushed motors. With brushless motors at higher timing settings all this goes out the window. If you look at the Silver Can graph below you can see the torque output is reasonably linear. Then look at the Orion Motor and you can see as the motor timing is increased the Torque output becomes much more non-linear.