You don't have to ice the bejeezus out of things to get the benefit of a chiller. Just recirculating 'room temp' water will give a nice increase in performance. I carry three 5 gallon water jugs in the trailer just for this.
As to how cold you can get things...it depends on the car. Generally, a carb'd stick car is going to have more latitude than a carb'd automatic.
I've built quite a few of these. For those wanting to do their own, make sure and pay attention to the size of the hose, fittings, etc. for good flow while keeping the pressure within limits. Many of the 110 volt pumps being used have flow rates too high to be effective at pulling heat out. And pressures well beyond what should be used.
Like anything, do your homework.
-Al