this has been discussed before and at some length too. the drop in temperature is going to be relative to the water circulating through the intercooler.... cooler water = larger temperature differential. automotive units for the most part are either air to air or liquid to air.

most factory turbocharged vehicles run an air to air intercooler. you will see a temperature difference relative to the ambient air temperature. most factory s/c vehicles such as your zr1 vette, ZL1 camaro, 03/04 cobra, cadillac CTS-V, ford lightning, shelby gt500 run a liquid to air intercooler with a heat exchanger up front similar to how a normal radiator works on a car. some of these you do have the option of adding ice to the resevroir to cool the circulating water down.

we circulate ice water through the car on each pass on the shop car.

some intercoolers are more efficient than others.... what it all comes down to is the water circulating through the intercooler.