The problem is not how far you can push it. It is how long you can push it for.

The 6 speed is more tolerant of slipping clutchs and limp mode then the 5 speed. You can push it real hard with lots of HP for short periods of time but you will not be able to continue to do this for long. It seriously shortens the life with somewhere around 90~100 HP over the stock HP. Sure the LML transmissions are stronger but so is the LML engine. That is why it is said to be "over stock". The more you push it the shorter the life. Higher the 90 or so and life shortens real fast.

It is also quite dependent on the tuning and how you dive it. Making 600 HP at 3500 RPMs is not the same as making 600 HP at 2500 RPMs. As you should know it is not really about the HP but the torque. The higher up on the RPM band you make the HP the lower the torque and thus the longer the transmission will last. Also keep in mind, driving a high HP truck like a grandpa, no offense to anyone, your stock transmission will last a long time. Drive it like a teen who just got their license and it sees the casket soon. There is no one HP level that is the magic number but it is well known with the pro tuners that it is close to 100

My 2.1 cents

 

I can see what you are saying. And working at a professional tuning company I can vouch that how hard you abuse a transmission will have a direct correlation on how long it last.
I do think it is worth noting that with our tuning, running 100 RWHP tune (Sport Economy) for the life of the truck is safe for any adult driver. Like you said, tuned or not, heavy abuse is going to damage a transmission, but driving reasonably will not hurt it.