It depends how sustained those temperatures are. As an example, 250*F for a few minutes is not an issue. 250*F all day is an issue.
I have had trans temp gauges on my last two trucks. In my 2002, the sensor was in the hot line out of the trans on it's way to the cooler. I saw 180 - 210* mostly. I got up to 250*F towing my RV climbing a steep narrow mountain grade with lots of pedestrian traffic at under 20 mph. It lasted about 15 minutes. No big deal.
The idiot light in my truck comes on at 280*F according to the research I've done, so that's where the folks that designed my truck think it is a problem. I monitor my temps in this truck digitally through the OBD2 port with a ScanGauge2.
2010 Cougar 322QBS 5er
2002 Dodge 2500 Quad cab, 4x4, 5.9L Cummins, 3.55, 47RE auto - SOLD
2007 Dodge 3500 SRW Megacab, 4x4, 5.9L Cummins, 3.73, 48RE auto 2014 RAM 2500 Crew Cab, 4x4, 6.4L Hemi, 4.10, 66RFE auto ** on order **