I'm sure you're right Dave. In engineering, there are certain things as "safety factors".
The TKO is rated by "continuous" torque, not peak torque. So if you put this thing on a dyno, you could make it hold 600 lb-ft all day long.
There are lots of things that we can do to increase the strength and shifting capabilities of the TKO as well. 700-750 lb-ft is very do-able with some REM polishing and cryogenic treatment of the gears, hubs, etc.
My advice to anyone that's in the market is to drive a car with each transmission. You may like one and not like the other. To me, the best shifting trans is a T5 or a Toploader. But the T5 won't last behind any power and the Toploader is only a 4 speed.