Its not practical idea at all and indeed a crazy one.We will explain why-----
When the AC is on , it takes away 50 percent of the total engine power.At the same time when you get part of the intake air from the cabin and part from outside, the AC takes much more time to get the cabin cooled as the intake takes away all the cabin cool air due to heavy intake ingestion [you might have noticed that when we put on our AC, and the cabin air circulation mode is on, at which the blower ingest and give out the cabin air only, the cabin get chilled faster than when we put the blower to external air ingestion mode], which is of higher pressure than AC blower outlet air pressure in the cabin, so there will be further reduction in engine power and what ever cold air will get into the intake pipe and fill up the air-fuel mixture will be nominal, so the overall power gain will be nil and may be less than standard condition with this idea for any given automobile.Hope it is clear to all.