This will not be efficient at all unless as stated previously you reach the design flow rate. Heat exchangers rely on specific Reynolds numbers inside of them in order to actually reach there design capacity. As to the amount of coolant that you use, it is almost impossible to tell because it depends on the different temperatures of your work and coolant as well as the flow rate through it.
As an engineer who works with these things all day y, I still think it would be much better for you if you sold it and use the cash to buy a smaller heat exchanger which is well within the design parameters of Wort cooling.
Mhb is right, you can minimise the losses with clever pipework and placement but Bigger is better does not apply to heat exchangers
