I've done this mod as well. I figured if I didn't like it I could just leave the lid on. Many are critical of it, and I was before I did it. Everything I learned in fluid dynamics seemed the contradict the idea of it working. More warm air from the engine compartment should reduce efficiency.

However, since peforming the mod, I do have a little more power, a 1 mpg increase in gas mileage, and it sounds throatier (though that alone wouldn't have been enough to keep it).

The only thing I can figure is that the increased quantity of intake outweighs the fact that some of the intake is now warmer air.

BTW, just cut through the grooves on the inside as well. The deckplate still went in fine for me. The grooves actually helped hold in the screen.