X-pipe on a straight 6?

[dvs_dave]

On a straight six with tubular equal length manifolds configured with two 3-into-1 collectors on cylinders 123 and 456, running separately all the way back, what would the effect of adding an x-pipe into the exhaust have?

Would it be advantageous or not really do anything other than maybe alter the sound? Does it improve exhaust scavenging at all?

[buggalugs]

People rekon so, most of the BMW 6's group the cylinders that way and have an X. Usually roughly under the drivers bum. I've seen no direct before/after dynos but people say the X gives you a bit more torque.