I tested the wort chiller (without the pre-cooler) by boiling about 7 1/2 gallons in my brew kettle, then cranking up the chiller. I measured the drain temperature of the cooling water and the "wort" temperature every two minutes until I got tired of it (and the "wort" temperature flattened out), which was about 20 minutes. The chart starts at 198 F because when I engaged the cooling water my inlet sprung a leak and I had to fix it. It took about 2 minutes.
The early cooling performance was good, but as the wort got closer to the cooling water temperature cooling slowed dramatically. I could probably dust off my thermo book from Memphis State and figure out why but I'm just not that interested. I think I should probably plan on using the pre-cooler all the time to create a bigger delta T so the wort doesn't spend so much time between 120 and 80.
Also, the garden hose I used to hook up the hot side of the chiller warped really badly while I was in "sterilize" mode before chilling started. I think I will swap it out for something with a bigger temperature tolerance.
My Mech E friend Mark looked at this profile and confirmed that I should be using the pre-cooler all along (while thinking "that's what you get for letting a software guy do cooling" I'm sure). I'm going to run another test this weekend with the pre-cooler to see what the effect is.