Recently I had headers from Xerd (www.Xerd.com) installed on my G35 sedan, they were made for the coupe, but fit just fine with no problems on my sedan. It truly is instant horsepower, you will feel it immediately. The sound difference was minimal with a little bit of a deep rumble using the factory resonator and muffler. I had an enlarged single input to a dual output muffler in the center of the car for duals in the back (removing the OEM resonator/muffler of course). It does add some extra power, and if you can handle the throaty sound, great, but I had to put the stock resonator/muffler back on for keeping it good for the wife.