I re-ran the wort chiller test today, and this time I used the pre-cooler in a water/ice bath to try to create a larger delta T between the coolant and the wort.
I attempted to duplicate the test conditions from Friday as best I could - I used the same volume of water in the boil kettle, I measured every two minutes, etc. There were some unavoidable differences though - one being that I had a dedicated probe for the wort temperature, I started measuring very near boiling (no leak to fix this time) and I was more rigorous on the timing of the measurements.
The result? No effective change in the time spent in the 120F-80F "danger zone." Here's the graphic proof. (You'll have to click through to be able to read it, sorry.)
I think the reason is simple. I measured the temperature coming out of the pre-cooler, and it was right at 63F. That was only a 3 degree drop from the supply temperature, which indicates that the pre-cooler wasn't doing much for me. I think I will have to get more tubing to build a better heat exchanger for precooling, but I'm not going to do it right now, because I expect my brewhouse supplies to arrive tomorrow and I need to get ready for the first batch. The cooling performance I'm seeing is (I think) good enough to get started.