What you say is true Steve. Exhausts are calculated to the specific needs of an engine. In this case that engine would be variated and this pipe would have a narrower powerband at higher RPM (Thus the loss of take-off power).
However, almost any pipe with some form of expansion works better than the (Heavily restricted) stock pipe (Not in all cases!). So this is basicly allowing the engine to breath easier and providing a lil pressure-wave boost that the old exhaust didn't have.
If you have an old expansion exhaust lying around, it's basicly free speed compared to stock. Beats buying a performance pipe if you run the rest stock.