I spoke with Doug Thorley hisself many years ago about this. He told me that all of they dyno and drag strip testing they ever did showed no appreciable gains from any kind of cross over pipe. Be it H, X or whatever. He also said (and I agree with him) that any kind of cross over takes away some of the crispness of dual exhaust.
I think for something on the street, you 'd not notice a power difference one way or the other, but the sound certainly does change with a cross over.......and IMO not in a good way.
