All I can say is that the Hobby Lobby ad on AXI motor is just nonsense, slightly misleading, they are talking about nitro RPM in the absurd range when they talk about USEABLE RPM (RPM too high as the plane's air frame will not meet the speed). That is pure hogwash, since when do we run a 46 2C at 15K-16K RPM?

There is no real formula to compare, motors are not the same as engines and EP props are not the same as our GP props. How many cells (volts) being used and what sort of battery (capacity and discharge rate) makes a lot of difference. Also, there is type of use, what sort of plane. What sort of gear ratio is being used. A lot of variables for EP. Then in EP it does not mean that the hotter the motor the bigger a prop it will spin (we do that in nitro engines), the amount of current flow is an issue, it can mean burning motors and ESC.

I am moving into EP a lot now but have kept it within the speed 300 and speed 400 sized motors and its really good fun. I find the power to weight ratio for size of plane in EP is more fun than nitro. However, the planes are made of foam and not balsa, and even if it is balsa its very flimsy.

I would say that your:

AXI 4130 with 6S lipoly is about the same as a Saito 100
AXI 2808 with 3S lipoly is about the same as 20 sized 2C
Speed 400 motor on 3S lipoly is about the same as 10 sized 2C

For me I find that the guide used by Hobby Lobby for AXI is a good one. Tells you what sort of weight the motor will haul. Of course I take the high end number as for the likes of a Piper Cub etc.