you ain't gonna see much gains with a dc header. The stock mani is designed very well. To see gains you're gonna have to spend and get a header with larger collector and cat or go with a torque stepped design like the comptech.

The most dramatic change you'll notice will a "standard" header is increased engine compartment heat. If your looking for mild gain and want to keep oem quality, I suggest you look into a type r stck mani.