Well my question is are they really two different motors and not just two identical motors with different firmware on the controller. The reason why I assume the ladder is with the bbs02, bbshd, m620 (I don't know enough about the others) even though they get listed as 48v and 52v motors the only difference is that firmware. The hardware inside is perfectly fine on the lower voltage models to run the higher volts. That when you think about it it makes sense bafang would do this as needing to redesign and manufacture two different motors and or controllers would cost a lot more than just different software.
Yes the same amount of power storage, but the discharge power of the 43v would be greater if both have the same amount of amps pulled from them. Say 15a, 36*15= 540w , 43*15= 645w. I'm not very familiar with the canbus stuff and how easy it is to mess with the firmware settings. I'm not super familiar with the m820 and what amps the controller pulls for the two voltages so this just an example: If at 36v say it pulls 15a or 540w peak, now at 43v bafang wants the motor to peak around the same so they limit the controller to 13a or 559w. You can see the controller is actually good for 2a more, if you find a way to unlock the controller you can get the full 15a. With previous motors what people would do if they had the lower voltage (like back when they didn't offer a 52v option only 48v) is that they would just run the higher voltage and it was fine, but I have heard some cases some firmware being set to throw a overvoltage error if it goes above the normal max. So people got in and either changed the firmware to not through an error, or when bafang came out with 52v firmware they got in and upped the max amps back to what it was with the lower voltage. Something might be possible with the m820 to do the same, getting the same amps as the 36v but with a 43v battery