The STX already has another external trans cooler in addition to the one in the radiator. So by adding the big Davies craig oil cooler you've actually got 2 external coolers then
i did realise you lose the warmup and the equalising factor when you no longer have the auto trans fluid running through the radiator, but as it never really gets below 0 degrees here i wasnt too worried
The trans fluid definitely doesnt get too hot with this arrangement , i did some sandy beach driving on a 40+ degree day, and had a suss of the fluid and it wasnt too hot at all