Tranny temps: how hot is too hot?

I have a very similar truck (2012 gmc). It's real hot here in central CA, I would see 200 on the flats at times. One of my tows is up a steep 12 mile grade, my transmission would hit 220 1/4 of the way up the hill. I would pull over let it cool 3-4 times before I hit the top. I'm sure it was fine but I didn't want to risk it.

I added an aftermarket cooler in line with the factory cooler. Now on that same Hill I won't even hit 200 at the top in 90-100 weather.