Well, like I said earlier I have pushed mine pretty hard and even through 100 yard 1-2 shift burnouts without any bad side effects other than snapping a rear shock off.:D
I'm sure my Magnum has easily twice the torque and horsepower of the 3.9
I have also pushed it in OD pretty good (120 plus)on the flat and over 100 up a 6% grade.
Don't even have an external cooler on it and I can tell by how it shifts that it doesn't get hot.