Not only do we have the input from an actual Ford trans engineer, but stop and think about it.
The trans cooler releases heat into the engine's coolant which then releases that heat through the - huge - radiator. That's a lot larger than any air-to-oil cooler you can put in it's (internal trans rad cooler) place.
Yes, adding an additional air cooler will help drop temps, but you definitely always want to leave the oem cooler in the radiator.