Cold air kit
Having purchased the replacement filter in 2011, I gave this a shot. The replacement filter gave an immediate increase in air flow and throttle response. But, getting the kit was an even better improvement than I anticipated. This along with a cam, headers and tune netted me 475 rwhp. Way to go!
Gave the car a throaty sound inside the cabin. Throttle response unchanged, mpg slightly improved. But now I've added a borla exhaust, shorty headers, and a new throttle body. Now averaging much better gas mileage. This isn't an amazing modification alone, but as a piece to the puzzle it's amazing, it really adds to the overall performance of the car when combined with other things