After 15+ years, the radiator is probably not all that great anymore (I'm in the same boat). I would ensure it's performing at 100% before adding coolers - or, just replace it. With your current TT there's no way you should need more trans cooling. Then add a couple of bottles of "Water Wetter" and change the coolant.
I would also have the trans serviced and have the shop verify the torque converter is locking up. If it doesn't, it adds a ton of heat.
If you end up with another cooler, don't go cheap. You really get what you pay for with them and what ever you do, don't bypass the stock cooler.
