I just did a test run with boiling water last night through my 10-plate Shirron chiller. Ground water was probably between 65F and 70F (I should have written it down, but I know it was not too cool). Draining the kettle at 0.5 gallons per minute and running water through the chiller at 6 gallons per minute (full bore from my washing machine supply) got 11 gallons down to 70F in 21 minutes. I was fiddling with output rates on the kettle and the hose, so I could probably bring the kettle rate down to about 15 minutes, just using a bit of gravity.
So, yeah, a plate chiller can get you really close to the incoming coolant water if you get it set up right as far as flow rates go. That's what I'm trying to determine now, as I want to get a pump to recirculate my coolant water instead of wasting 100+ gallons.