There is a science that goes into designing Y pipes and X pipes.
You want to use a Merge collector style, such as Vibrant or Burns stainless design in my opinion when going with a Y pipe such as below.
And for an X pipe, this design has different inlet and exiting taper degree angles. Which as stated above is essentially two 2-1 merge collectors mated together.
These designs above are what you want.
You don't want those siamesed or extreme wide angle X pipes such as these styles below...
"The book, "Four Stroke Performance Tuning" has a section on exhaust where the author, A. Graham Bell, explains the design of the venturi merge collector. The V2 section is two of these merge collectors joined together at the constriction point. The inlet pipes merge at an angle of 9 to 12 degrees each for an included angle of about 20 degrees. A scan of that page from his book illustrates the design. My point is that when experienced exhaust designers design a Y merge, or V2 section, they do not use the wide angle V2 or siamesed bend sections joined on the outside"
Hope this helps some, if you guys are into the technical aspect of it all.
In my opinion though MOTORDYNE engineering has got it right for the 350/370z crowd.
They know what they are doing when it comes to exhaust setups.