We may have to agree to disagree. Efficiency of an engine can be defined such that "more efficiency" is "uses less fuel" or efficiency = distance traveled/volume fuel consumed. An internal combustion engine is not at its most efficient when the throttle is wide open, though that is when it generates the most power. You could perhaps define "more efficiency" as "faster" or efficiency = distance traveled/unit time, but most would call that "speed".
If you're defining "more efficient" as "uses less water" or efficiency = heat removed from wort/volume water used, then the most efficient thing to do would be to let your wort air cool and forget the chiller.
But if you're talking about heat transfer efficiency, which I think both Andy and gmac were, then "more efficient" is defined as "faster cooling" or efficiency = heat removed from wort/unit time (which is really "rate of heat transfer" I suppose). "Heat transfer efficiency" would really be rate of heat transfer achieved/theoretical maximum, but why bother with the denominator? But I think the water use definition is equally valid.
Sheesh, that was boring. I need a homebrew.