I have done this on several cars previously. MPG's will improve when you are cruising. It's hard to measure HP increase since on a dyno the car isn't moving. But look at it this way, a well designed system never looses power. As above a true "Ram Air" is more complicated. Most people use an inlet near the bottom of the bumper. This is a good source of fresh, cool air when the car is moving. Make sure to use an air filter that can get a little wet, like K&N. Also filter maintainace is greatly increase. You will pick up a lot more dirt if the inlet is near the ground. Check/clean/replace the filter every six months. Watch out for deep puddles! There is an increased chance of hydrolock with an air inlet that close to the ground.
