There are people with over 1000 hp going through the stock internals of the TR6060 with it still holding up (obviously upgraded clutch). In our cars I'm pretty certain the diff and axles would go before you have to worry about the transmission.
It also depends a lot on how you drive it too, I found this on Tremec's site:
"Our T-56 Magnum 6-speed is rated to accept as much as 700-lb. ft. of sustained torque in a vehicle weighing as much as 4,500 lbs. However, in a case where warranty considerations are not as great of a concern, it is important to note that TREMEC Transmissions can often handle substantially more power if the user practices common sense and understands the failure conditions of a given driveline part. For example: A four-thousand-pound-plus car launching at high rpm on drag slicks is more likely to cause a failure than a lighter weight, smaller displacement, street car on aggressive street radials."
