In my Zen there is no thermostat to control the temperature inside the cabin. If I move the slider to the right it only mixes warm air with the cold air, still keeps the ac compressor running. There are sensors that cut out the compressors on high/low pressures. But not thermostat to control the temperature inside.
You have said that you get better FE if you keep it in the middle and that the engine response is better. If you are getting cold in in your airvent , it means that the compressor is on. Is your engine response fluctuate? if the ac is cutting of and on you should feel that.