You can read the entire article at http://www.fordmuscle.com/archives/2000/04/flowtech/index.shtmlX Versus H
It's long been known that connecting the two sides of a dual exhaust system, shortly after the headers, increases power and reduces sound. However in a traditional "H" pipe, only sound and pressure waves cross through the connection. Gasses take the route of lowest pressure, which is straight out the pipe, rather than making a 90 degree turn in to the H.
Indy and Nascar teams have been using "X" type crossovers for quite some time. In an "X" pipe the gasses from both cylinder banks actually pass through a common area. This not only provides the benefits of some sound wave cancellation, but also increases the scavenging effect, helping draw out spent charge from the combustion chamber.
So how much of an increase does an X gain over an H? It's hard to say, especially considering that every combination will yield varying gains. In theory an X will work better as rpms go up. Considering the Flowtech X- Terminator pieces are priced under $50; you're much better off getting that, rather than having a shop fab up a crude 'H' piece.