I live at 4k feet in the mountains and we see temps in the 10-20* range in the winter. From the research I've done I think you will be ok with those temps (40*) in winter. The only people who need to be concerned are the ones living in climate zones that see 0* or less.

Overcool a transmission. - Bob Is The Oil Guy

Also, a recommendation I found on some sites says to place a piece of plastic or cardboard over the cooler in winter to prevent some of the airflow so it stays a little warmer. You're only going to know if you have a temp gauage. I would pick up one of those as well just so you can see where you're at and make a decision from there. From what I've read, you want your Tranny temps to stay between 150-180*. The scary thing is that the stock radiator (heat exchanger) keeps your temps at around 200* due to the temp of the motor.

Some of the newer tranny coolers have a LPD (low pressure drop) line which bypasses the cooler in cold weather.

My cooler doesn't have this so I will be watching the Tranny temps on my ScanGaugeII to see if I need to cover part of the cooler in winter/snow temps. I would rather deal with that minor inconvenience each year than worry about the strawberry milkshake of death!