"H-pipe," X-pipe," or no pipe?

I know that the factory "H-pipes" were a great improvement over separate dual exhaust pipes. New data seem to indicate that "X-pipes" are an improvement over "H-pipes." However, are these pipes of any benefit over headers with only open headers and mufflers? Do they provide benefit only with full exhaust systems?

Am I correct in noting that Super Stock cars run with "open" headers with only an extension pipe and no crossover pipe?

The reason I ask is that I plan to use header mufflers and side exhaust on my '68 'Cuda.