They do not flow better than a quality 4 tube header but, up to a certain hp level they do not hold you back. Cam profiles make a difference here too. Good advise to avoid the headers until you you know you need them.
As mentioned the RARE longbranch manifolds are excellent pieces. Make sure your heads have all the needed exhaust bolt holes drilled and tapped. All D-port heads use the same pattern but on some heads not all the holes are drilled and tapped. The boss may be there but no machine work done.