Does anyone know if the captiva boxes have a thermostat controlled cooling setup like some other transmissions do?
If it does, it wont allow flow to the cooler untill it reaches "operating" temperature anyway. If so, you can safely run an external only cooler if you want to.
If it doesn't control flow to and from the cooler, then it might be useful to have the factory one still in the cooling path.
As a rule of thumb i've always based whether the external cooler should be first or last should be dependant on where you live, if always cold climate but heavy towing, internal first and external 2nd.
vice versa if in hot climate, use the external to knock the heat out first and then hopefully the cooler tranny fluid will help the engines cooling system maintain a lower temp by pre warming the fluid to operating temp before returning to the box.
The rate of flow of the transmission fluid i believe should be fast enough that the much higher operating temp of the engines cooling system shouldn't raise the transmission fluid by much.
Something to have a think about....
The coolant in the radiator doesn't sit at all times at the same temp as the engine.
Think about it, the thermostat only opens when it reaches temp and then shuts again when temp falls below it's threshold.
Therefore, when the thermostat is shut, the coolant in the radiator that is still getting airflow to cool it down will always be cooler than the engine.
There may be a difference of 5-20 degrees between the coolant cycling in the engine vs the coolant held in the radiator.
Dunno if my conclusions from 15 years of tinkering is of any help or not...