here's my understanding of the topic.

New, high quality torque converters are good for the street, AND will still stall high on the track for good ETs.

remember, it's a TORQUE converter. TORQUE being the key word, not "RPM"

the more torque you put into it, the more it will slip. at maximum torque, you'll get maximum slippage, up to the stall point.

under part throttle, light load cruising, your not really making any torque, and that "high stall TC" is not slipping excessivly --again, at part throttle cruisng.