Heat flows from warmer to colder, and the amount of heat flow is based on the gradient (i.e., temperature difference, with a bigger temperature difference meaning more heat flow). Under light throttle/cruising conditions, the TCC is locked and the trans will generate very, very little heat. At this period the engine should be running around 195F, resulting in the coolant transferring heat to the ATF and warming it up to around 195F as well. I don't think the CSF would change trans temps under this conditions the engine temp is controlled by the thermostat and should not be affected by the size of the radiator.
When the ATF exceeds engine temp (which usually happens when the engine is working hard), if the coolant temp were lower than it would be with a OEM radiator, due to more radiator capacity (i.e., the thermostat is wide-open as the system temperature is controlled by the radiator/fan cooling capacity), the trans temp should be lower with a CSF radiator as the temperature gradient between the ATF and radiator will be higher - thereby allowing the ATF to shed more heat to the radiator.
I still recommend an aux trans cooler on any of these rigs. Mine is installed after the OEM radiator/trans cooler (OEM location for a GX470, though I have a larger-than-stock cooler). The aux cooler is not combined with the radiator, so it allows the ATF to shed additional heat directly to the atmosphere, which will always be cooler than the 195F coolant temperature. In theory this should also help with engine temp as aux cooler shedding heat to the atmosphere means less heat is being transferred to the engine coolant.