If the hose material of your previous cooler lines was indeed originally designed to be used for fuel vents, then somebody has done a bad engineering job. Trans fluid lines are specific to that application, to handle hot hydraulic oil at under a hundred PSI.
OEM engineering department's experience has shown that you get a LOT of cooling from the trans cooler that is part of the bottom radiator tank. The bottom tank is not the one at 200 degrees, BTW, and that cooler is pretty effective.
However, the addition of an air-to-oil unit can only be of additional benefit, UNLESS it slows necessary flow. Stack plate types are superior to fin-and-tube types in that regard.
Your new cooler (heat exchanger) is completely dependent on air flow, which isn't all that terrific in stop-and-go traffic or long mountain passes behind slow trucks. Now, couple that with hot ambient air temps, and you may find that there are situations where you will have done your transmission a disservice, temp-wise.
There are other scenarios, like over-cooling, too, but I suspect that's beyond the scope of this discussion.
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