SVT Performance did some comparison testing years ago and CAI's by themselves had small gains, 10-15 hp on the dyno runs. The principal behind CAI's is to allow more, cooler, less restricted (cleaner) air flow into the throttle bodies and into the engine ports. Stock/OEM cars are set up appropriately with air boxes, TB's etc. for what the engine needs, at their birth. As you modify the engine, you may need to upgrade the other components in order for the engine to operate at peak performance.
IF MY CAR WENT AS FAST AS THE MONEY I SPEND ON IT, I'D WIN EVERY RACE