An X or H pipe will most definately produce more power than just 'true duals'. This has to do with the nature of a 90* V8 engine. In the normal V8, two cylinders on the same bank will fire within 90* of each other, creating excessive exhaust pressure in that bank's exhaust system. By installing a crossover pipe, this excess pressure is diverted to the other side's system, thus reducing backpressure and promoting horsepower.
car companys, F1 racers and nascar wouldnt be spending the money on them if they didnt work.
3spd got it right on the nose. An X and H pipe baffels (balances) the exhaust out, which give better flow. An X-pipe is more efficient then an H-pipe, but most poeple say that an H-pipe sounds better then an X-pipe, so you'll have to decide which is more important to you.