Has anyone installed an oil cooler, or transmission cooler (Aftermarket)

Not sure about the G3 and beyond but the G2 Inverter coolant lives about 100 degrees cooler than the ICE coolant so very good temp conversion
There for the trans and inverter.

The G3 and beyond got a big inverter coolant improvement with a separator inverter coolant heat exchanger where the G2 HE was in the same Rad as the ICE so lots of heat soak there.

So big cooling improvement there. Probably became necessary with the much more powerful trans motors and inverter. Much more powerful car.

 

I think neodymium is ok up to around 210° or so, so it's probably never that hot in the transaxle.

 

But when you are using all that awesome EV power in the Prime, where do you think all that wattage in the stators play against?
(how much is it?)
Hint. The rotor magnets get hot in service and that is part of the ATF cooling duties.

 

If they get hotter than around 210°F and keep their magnetic strength they aren't neodymium. Maybe they're using some kind of Samarium-Cobalt magnets? If you have a neodymium magnet on the bolt and it's exposed to pronlonged temperatures above 210°F it will permanently lose magnetic strength.