K&N official statement on their website
K&N AIR INTAKES, THE ULTIMATE WEAPON IN THE HORSEPOWER WARS
Most of our High-Flow Air Intakes can be purchased for less than $350. On the 2003 Ford Mustang SVT®, we actually achieved a 30 horsepower increase. Even our typical air intake gains are in the range of 8 to 16 horsepower. To send an average V-8 down the road at 55 mph, it takes about 17 horsepower. Considering this, you begin to realize just how much of a difference our intakes can make when you step on the gas. We’re proud of these intakes because after you bolt one on, you’ll know something’s different…VERY DIFFERENT!