I bought a McPappy in 2012. Flywheel was included. Nice piece of kit I must say. I did make up various collars etc on my lathe to help with the different layouts. Bought an Eagle tree datalogger, sensors and digital readout panel.
Just an opinion but whilst results were interesting, personally I did not think they were really all that helpful with regard to on road circuit racing, for me at least. One of the reasons I felt was because the way electric motors make their power is rather load dependent and this varies a lot around a circuit, and from circuit to circuit.
I did read a lot of helpful posts by John Stranahan some years back. He makes a very good case for oval racers to use the dyno however.
As I had the gear, I placed the Eagle Tree system in the car and went out on the track with it. Now that was a revelation. Gives a good insight into how boost affects current draw, and therefore how much you can use, and how quickly it can be applied before it just turns to heat. This is good to know.
When you have enough data to correlate into something meaningful it is possible to interpret the readings from an ordinary motor tester very effectively. So I just use an Extemerc tester from RCMarket or whatever (quite cheap) and a Fantom Facts Machine Jr gauss meter these days. TBH I hardly ever touch the settings now. The rules mean that overall motor performance is very similar between brands once you're in the sweet spot.
Sadly for me tuning the car and driving it are much more important to laptime