Lol maybe in the rags they claim those numbers. If all the other "supporting mods" are done you'll gain 10-15whp at the most from going to longtubes/shorty mid pipe.
The Terminator guys who know what they are doing generally don't swap the stock manifolds out until they move up from the Eaton to a KB/Whipple if that tells you anything. And as far as brand, Kooks or ARH or gtfo and for that price there are likely other things you can do that will add more HP. Heck, even finding an Eaton swap for a 99/01 Cobra can be done for the $1k-$1500 mark which is about what it will cost for the Kooks setup.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.