Well, lockup generally happens in 4th gear in an automatic. Some programs for the 4L60e can force the lockup clutch in the converter to engage (when it ordinarily would not) for dyno runs but this is not a common modification. Stall speed is determined by several factors, including the distance between the impeller and the turbine and the design of the stator. It is not a matter of "throwing the extra power away as heat." If you think of the design of a stall converter as two fans facing one another with a fluid transfer rather than an air transfer, you're getting the idea. Going further, the angle of said blades are what determines the stall or what manual transmission guys would say is the "point of engagement."

That being said, it is not a heat-loss transfer issue, it is a controlled engagement point. Granted, I can see why you'd think it was throwing away the extra power due to heat because the friction of the fluid moving through the converter and throughout the transmission itself causes heat. This also occurs in every automatic transmission on the planet which is why you'll never see an automatic without a heat exchanger.

I'll quit now, I'm no longer a transmission guy...that was a former life.