northern aluminum radiator

I don't think the radiator will cause the tranny temps increase. Many guys on a truck website that I am apart of have tranny gauges and high stall torque converters. They use both the radiator and a secondary cooler. there tranny temps stay around 150-160 but the thermostat the same trucks is a 195. I think the radiator part mainly just stablizes the temp variation. Not too cool and not too hot.
 
I have a hard time believeing the radiator can pre-warm the tranny fluid. Remember, radiators don't gradually warm up with the engine. They are at ambient temp until the stat opens, then they are considerably warmer right away. Since it would take, theoratically, 3-5 minutes before the stat opens,. I would say the tranny is already well warmer than the coolant before the stat opens.

That would be my guess, anyway....