Hi-Po Exhaust Manifold Performance?

I have a 68 stang with a late model GT-40 engine. I currently have full length headers, but am thinking about replacing the headers with the early model (1967) Hi-Po exhaust manifold that was used on the k-code 289s, and then using a standard dual exhaust. Has anyone had experience using this manifold in a true performance application?

thanks. i figured i would lose some power, do you have anything quantitative on how much? i assume that the hi-po manifold is better than stock.