The hfc will rob a negligible amount of power from the car with a good header.
If I were in your shoes I would go with an SVT header and a modified R/T cat to bolt up to a Zetec flex. That will make the most power for you. A Short header and a cat in an L pipe will be more restrictive.
If I were in your shoes I would go with an SVT header and a modified R/T cat to bolt up to a Zetec flex. That will make the most power for you. A Short header and a cat in an L pipe will be more restrictive.