On my '12 it's definitely an additional heat shield as opposed to a replacement. But, no, on an '04 I don't know if that's the case.
There was certainly no noticeable difference in performance when I had my SPT shield added. If there's no noticeable difference, then in my book it's not a benefit. I was kinda' disappointed.
EDIT: Before buying the SPT shield, I scoured the web looking for definite proof, or at the very least someone who could claim they noticed anything change after having it installed. I found nothing. Nada. Zip. Of course, the service guy at the dealership said it undoubtedly made a difference, but couldn't point me to any info confirming that. I figured I'd try it anyway because it was cheap and couldn't do any harm (other than add weight).