That is the Fantom site talking about their own pipes. Does what they say hold true for pipes in general?
I know the size and shape of the convergent and divergent cones also make a difference. I used to have a link to an article that explained how and why a tuned pipe works, but it's a dead link now. It was a good article with diagrams and everything.
Length of the whole system is also a factor to consider. I have read about how you can trim off the end of the header(the pipe end, not the manifold end) to shorten the system to time the pulses.
As a general rule...as I understand it...shorter pipes with longer divergent cone/short barrel/short convergent cone are suited to high RPM.
...long pipes/long barrel/short steep cones are suited to torque and lower end power.

Once you take into consideration all of the variables and do some figuring and calculations...get the one that sounds cool.