Its good for sound gains, slightly cooler air, and slighly less restriction resulting in about 5whp on an untuned car (tuned expect up to at least 15whp). As it is, the stock Intake (which is technically a CAI) on the STi is pretty good. Some aftermarket CAI's mess with the MAF signal which will cause a lean condition. Luckily, the APS CAI is one of the good ones (Injen is another) that doesn't and subsequently doesn't require a tune.